HRSG Drum Level Control in Combined Cycle Power Plant Using Fractional Order PI

Abstract

An important issue in the combined cycle power plant operation is the control of drum water level in the boiler. Under steady operating conditions, the drum level is usually stable. However, for power plants that are frequently changing load or subject to sudden disturbances, in such circumstances poor control can result in costly plant trip or even serious damage by boiling the drum or carrying water over into the steam turbine.

Accordingly, in the Thesis, the combined cycle power plant main components are introduced as well as the main control loops for the Heat Recovery Steam Generator (HRSG). The HRSG drum level control is studied. A MATLAB model is developed to simulate the heat flow rate for four gas turbines feeding four HRSG drums, and four HRSG drums steam flow feeding two steam turbines control valves through common steam header, as shown in Figure (4-1) and Figure (6-1). The steam turbine control valves are operated using Sliding Mode pressure control.

The drum level controller is a three Element drum level control with cascaded and feed-forward control. In the thesis the disturbance is represented as one gas turbine trip,and the drum level of the other three HRSGS is controlled using conventional PI controller. Then Fractional Order PI Controller is introduced to the closed loop drum level control.

It was proven that the fractional order controller utilized to achieve drum water level control of HRSG, gives better results: less settling time, undershoot and overshoot, under different disturbances compared with conventional PI controller.