الفهرس 
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Abstract
An important issue on the combined cycle power plant control is the control of drum water level in the boiler. Under steady operating conditions, the drum level control 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 cycled power plant main components are introduced as well as the main control loops for the Heat Recovery Steam Generator (HRSG). The Thesis will study the HRSG drum level control. The MATLAB model will simulate the heat flow rate for four gas turbines feeding four HRSGs drum and four HRSGs drum steam flow feeding two steam turbines control valves through common steam header. 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 feedforward control; the thesis will illustrate the disturbance of one gas turbine trip in the drum level of the other three HRSGS using classical PI controller. Then fractional order PI Controller is introduced to the closed loop drum level control. It is proven that the fractional order controller utilized to achieve drum water level control of HRSG, achieves better results with less settling time, undershoot and overshoot under different disturbances compared with classical controller