Anti-disturbance nonlinear control for wide-load operation of ultra-supercritical units using high-order sliding mode observer

Ultra-supercritical (USC) coal fired units in China are required to operate in a wide load range to absorb intermittent renewable energy generation into the power grid, but various disturbances deteriorate the control performance of coordinated control system (CCS), further threatening the operational safety and flexibility of USC units. To this end, this study proposes an anti-disturbance nonlinear control method using high-order sliding mode observer (HOSMO) to develop the CCS of USC units. Firstly, the setpoints of CCS are designed by using its dynamic characteristic and variation rate of steam temperature in separator to avoid the deterioration of safe and flexible performances caused by load disturbance. Secondly, to reject various disturbances, the HOSMO is combined with the CCS model and proportional and integral control to design the feedforward, feedback and disturbance compensation control actions, and then coal flow in furnace can be accurately estimated online to reject the disturbance of time-varying delay. Lastly, stability analysis and simulation tests are performed. Simulation results reveal that the proposed method has excellent tracking and anti-disturbance performances by comparison with error active disturbance rejection control (EADRC), phase lead error-based active disturbance rejection control (PL-EADRC) and conventional proportional-integral-derivative (PID) control. Its root mean square errors of system outputs are 1.5∗10−3 MPa, 1.6∗10−3 kJ/kg and 1.7∗10−3 MW, respectively, and the tracking errors are reduced by at least 80 % compared with other control strategies. Hence, the proposed method can provide reference for improving operational flexibility and safety of USC units.