Using Linear Control Theory for Parameterization of a Controller for a SOFC/GT Hybrid Power Plant

The purpose of the current work is to develop an appropriate control system for a solid oxide fuel cell/gas turbine hybrid system. The main focus lies on the usage of linear control theory for the parameterization of the controller structure. The studies are carried out with a control oriented simplified dynamic model of a 25 MWe hybrid system based on a conceptual design previously presented in literature. As a specific feature additional firing of the gas turbine combustor is used as an extra actuating variable offering several advantages. Foregoing necessary investigations deal with calculating the on- and off-design behavior of the sample system and with defining a reasonable part-load operating curve taking into account several constraints like efficiency, stack temperature, surge margin, etc. Also, preliminary studies of the transient open loop behavior are performed. They reveal that the input variables should be changed with specific care to avoid critical situations during load change. As a precondition for linear control theory a low-order linear model is deduced and validated. With consideration of the transient simulation results and of the properties of the linear model a proper control strategy is suggested, which consists of a proportional output feedback taking into account the multi-input-multi-output character of the system and three distributed proportional-integral controllers that define the requested load point onto the operating curve. Lastly, the controllers are parameterized based on linear control theory and verified.