Fixed-Time Fractional-Order Sliding Mode Coordinated Control for Boiler–Turbine Systems with Disturbances and Input Constraints

In coal-fired power plants, the coordinated control of boiler–turbine systems (BTSs) is required to track grid load dispatch instructions in real time while maintaining stable main steam pressure. Nevertheless, a BTS is affected by input constraints, external disturbances and model uncertainties, making it challenging for existing control schemes to stabilize the system within a fixed time. To tackle this issue, a fixed-time fractional-order sliding mode coordinated control (FTFOSMCC) strategy is put forward for the BTS. Firstly, a novel fixed-time fractional-order sliding mode disturbance observer (FTFOSMDO) is constructed by merging Riemann–Liouville fractional calculus with the fixed-time stability theory; the proposed observer is theoretically proven to drive the disturbance estimation error to zero within a fixed settling time that does not depend on the initial observation error. Secondly, the FTFOSMCC controller incorporates the Caputo fractional-order sliding surface, a novel variable exponential power reaching law, the state-of-the-art anti-windup technique, disturbance-observer-based feedforward compensation technique and fixed-time stability theorem, guaranteeing active suppression of actuator saturation and fixed-time stability of the closed-loop control system by Lyapunov’s theorem. Thirdly, simulations on a 300 MW subcritical BTS validate the exceptional tracking accuracy, disturbance rejection capability and robustness of the proposed strategy with the reduction of 33–90% in maximum tracking error, 12–98% in cumulative error, and 11–98% in time-weighted error compared to tuned PID and integer-order disturbance-observer-based sliding mode controllers.