Nonlinear dynamics and control of bifurcation to regulate the performance of a boiler–turbine unit

The economical operations of power plants and environmental awareness are the major factors affecting the importance of control in boiler–turbine units. In this paper, a multivariable nonlinear model of boiler–turbine unit is considered. Drum pressure, electric output and water level of drum (as output variables) are adjusted at desired values by manipulation of valve positions for fuel, steam and feed-water flow rates (as input variables). Nonlinear dynamics of the unit is investigated through the concepts of bifurcation and limit cycles behaviour. In the presence of harmonic disturbances, some coefficients of the dynamic model, fuel and steam flow rates play as the bifurcation parameters. It is shown that variation of bifurcation parameters leads to the occurrence of secondary Hopf (Neimark) and symmetry-breaking bifurcations (in drum pressure or electric output). To improve the aperiodic and quasi-periodic behaviour of the system to the stable periodic one, a control regulator is designed based on feedback linearization approach. It is shown that the controller acts efficiently in disturbance rejection and guarantees the stable periodic solutions with low oscillatory behaviour (which is essentially demanded by the power grid).