FUZZY APPROACH TO BOILER DRUM LEVEL CONTROL

Abstract

The drum —boiler system is an integral component of a subcritical thermal power station and water level (& steam pressure) is one of the main control parameter in the steam generating unit. Drum level control is difficult because of the complicated swell & shrink dynamics. This creates a non-minimum phase behavior which changes significantly with the operating condition. The 3-element proportional-integral-derivative (PID) control is a popular conventional approach for drum level control. The control scheme works satisfactorily in the absence of any process disturbances. However, performance of this controller is limited since they don't account for variation of system parameters, and periodic tuning is necessary. So, there is a good motivation to investigate alternatives to this control scheme. Since the boiler is a nonlinear, time varying multi-input multi-output system whose states vary with the operating condition, so for such a system fuzzy control has been chosen in this work. For drum level control, fuzzy-PI cascade control has been used where fuzzy controller acts as master controller in the cascade control. Another fuzzy logic controller has been used to regulate drum pressure at desired value. The control scheme has been simulated on 160 MW power plant unit on the Swedish grid system. The drum—boiler model has been implemented in the MATLAB 7.01, simulink. The control scheme has been tested for different operating condition (high & low load operation) and also tested under different input fluctuations (Steam flow, feedwater temperature &feedwater pressure fluctuation). A detailed analysis on the basis of parameters like root mean square error (RMSE),maximum overshoot, maximum undershoot has been carried out to evaluate the control variable responses under disturbances. Simulation results have shown that the control scheme's performance for wide range of operating condition and disturbances is especially desirable