Letting off steam (Sistema de control de agua de alimentación) A modification of a conventional feed water three-element control strategy to solve problems of drum level control

Acontrol strategy for the drum level control of a combined cycle power plant is presented. This strategy is based on the conventional three element system with variable proportional gain in the master and slave controllers. The dynamic behavior of the proposed strategy is compared with a former one showing a better answer especially at full load (generated electric energy) with the supplementary fire (duct burners or after burners) in operation. When the feed water control valve is near its full opening, this strategy can avoid as much as possible the saturation of the control signal to the valve, with elimination of the permanent oscillation of the controlled variable (drum level). The control system behavior also shows good stability from start up to full load, including the transition from one to three elements operating mode and vice verse, as well as the duct burners’ ignition. Similar stability is shown during the stop of the Heat Recovery Steam Generator (HRSG) from full load with duct fire in operation to the HRSG out of service. Due to the performance of the blocks communication software of the commercial equipment in use, a transition band of the steam flow signal is proposed, in order to assure a three to one or one to three element bump less transfer operation. Simulation laboratory tests were applied to the proposed control strategy with a simplified dynamic process model, before the control system was put into operation in site. Real process runs and the control-tune parameters are presented.