| A twelfth order, physically based, nonlinear boiler-turbine model of a 360 MW coal fired power plant is developed for use in control system design. The existing computer controlled feedback and feedforward control system is modeled in the normal operating mode for use in validating the process model. The process and control system models simulate the plant well in the normal operating range of 160-360 MW in both steady and transient conditions. However, the limit cycle with a period of about 6 minutes observed in quasi-steady state data at near rated load cannot be reconstructed in the model mainly due to a lack of information. The inferred reasons for the cause of the limit cycle are presented. The linearized process models at various load levels are found to approximate the nonlinear model quite well. Using the Inverse Nyquist Array method, a new multivariable control system is synthesized and simulated with the same MW ramp changes used in the Linear Operations Test conducted in 1981. The proposed control system responds much faster with less than 25% of the deviations in the critical output variables observed with the existing control system. Also the load rate is improved by approximately three times, from the current 7.8 MW/minute to 21 MW/minute. The new control system has the advantage having the structure similar to the existing control system and retrofitting the control system is relatively easy. Some hardware changes and further research efforts to improve the plant response speed are suggested. |
