Modeling and predictive control of a nonlinear power plant reheater with switched dynamics

This paper is focused on modeling and predictive control of a power plant reheater. The motivation for this research is the necessity to achieve tighter power plant control in the wide range load following operation. The reheater under consideration consists of a series of three heat exchangers and two of them can be fully or partially bypassed. Accurate first principles model of the reheater is developed in the first part of the paper. Good approximation of the distributed parameter behavior is obtained by dividing the dynamics of the individual heat exchangers into several control volumes. Resulting high order model is considerably nonlinear. Moreover, switching of its dynamic behavior occurs because of the bypassing of the individual reheat stages. Thus, this reheater represents a non-trivial control challenge. Model predictive controller is designed in the second part. It is based on a piecewise affine model. Piecewise affine model is used both to approximate reheater nonlinearity and to describe the switches of reheater dynamics. The performance of the predictive controller is compared with the classical control system. Considerable performance improvement can be observed.