Gain-scheduled nonlinear control of U-tube steam generators at low powers

A nonlinear gain-scheduled controller for the water level of a U-tube steam generator (UTSG) (Westinghouse type), which covers the entire operating envelope and exhibits improved performance at low operating powers, as compared to existing controllers, has been designed. Several numerically linearized models of the UTSG were developed using a validated nonlinear model that covers its entire operating envelope. The linear quadratic Gaussian with loop transfer recovery (LQG/LTR) method was used to design dynamic compensators for each of the linearized models. The various compensator matrices were fitted to a scheduling variable, namely, the temperature difference between the primary side hot- and cold-leg temperatures, resulting in a gain-scheduled nonlinear compensator. The performance of the gain-scheduled compensator (GSC) is systematically investigated via transient response simulations using the nonlinear UTSG model. The GSC performance has been found to be satisfactory for both step and ramp changes in power demand. It has also determined that the designed GSC exhibits more robustness to certain modeling errors than alternate more complex controllers suggested in the literature