Robust FOPI controller design for power control of PHWR under step-back condition

•Robust stabilization of a highly nonlinear PHWR process.•A robust FOPI controller for a typical fractional-order NIOPTD-II plant.•Flat phase constraint is proposed to obtain a controller giving wide flat phase at gain crossover frequency.•FOPI controller designed for an operating condition of the PHWR produces a constant Phase margin for wide range of frequencies.•The proposed FOPI controller gives better performance compared to existing in the literature.

This paper presents a stabilizing fractional-order proportional integral (FOPI) controller design for the power control of a highly nonlinear Pressurized Heavy Water Reactor (PHWR) under step-back condition. A single robust FOPI controller is designed utilizing stability boundary locus method for eight nuclear reactor models of the PHWR linearized at different operating points. A set of stabilizing controller parameters is obtained for a typical fractional-order two non-integer order plus time delay (NIOPTD-II) model of the reactor satisfying design specifications of phase margin and gain crossover frequency. Then a flat phase constraint is developed to find a controller giving wide flat phase at the desired gain crossover frequency. The controller obtained is found to give iso-damped closed-loop response for all linearized models of PHWR. Simulation results show that the proposed FOPI controller applied for active step-back in the reactor give a deadbeat tracking performance without any undershoot for all operating points, defined for variation in initial power level or control rod drop. The efficacy of the proposed FOPI controller is verified by comparing its performance with existing methods in the literature.