Fractional-order sliding mode coordinated controller using super-twisting disturbance observer for an NSSS with predefined-time stability.

This paper develops a fractional-order sliding mode coordinated control (FOSMCC) strategy incorporating dual super-twisting disturbance observers (STDOs) to enhance the control performance, stability, and reliability of the nuclear steam supply system (NSSS) under complex, time-varying operating conditions and compound disturbances. The FOSMCC strategy synthesizes the fractional-order control and predefined-time theory with sliding mode control, augmented by the disturbance feedforward compensation loop driven by dual STDOs. Such control framework provides enhanced control performance guarantees, including fast transient response, high steady-state precision, and reinforced disturbance rejection. Furthermore, by employing Lyapunov's direct approach, it is theoretically demonstrated that the entire NSSS, under the developed coordinated strategy, achieves superior predefined-time stability. Finally, comprehensive numerical validation and comparative studies reveal that the developed FOSMCC strategy with STDOs significantly outperforms both the latest fractional-order fixed-time sliding mode controller (FOFTSMC) and the practically adopted coordinated controller (PACC), exhibiting better transient/steady-state control response and stronger robustness against disturbances. Simulation results validate that, in the presence of compound disturbances, the proposed FOSMCC strategy reduces the integral absolute control error of nuclear power and water level by 89.37 % and 87.67 %, respectively, compared to FOFTSMC, and by 99.97 % and 99.99 %, respectively, compared to PACC.