On the Management and Control of Isolated Power Systems
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Power systems have been a rich arena for research and study since the very beginning of their use commercially in the late 19th century. At the beginning, the objectives of the research were focused on the stability and reliability of the power systems, especially with the fast growth in their size and in the demand on the electric power. Later, due to the need to save losses and fuel consumption, the increasing awareness of the environmental challenges, the growing desire to integrate renewable energy resources, and many other reasons, the attention of the research was drawn to another essential topic, that is the management and planning of power systems. Isolated power systems, however, did not receive similar attention until recently. Isolated power systems are indispensable in many applications such as, but not limited to, island power systems and marine vessels. This work addresses two different topics in the study of power systems that are intended for isolated power systems, but the results provided can also be expanded to regular ones. Thus, the thesis is divided into two parts: the management and scheduling of the power systems, and the control of Gensets. The first part contains our contributions in the field of operations research and the management of power systems after providing a brief introduction to integer programming (IP), mixed-integer programming (MIP), and propositional calculus. This introductory chapter is fundamental to understand the basic concepts that were used to develop the models and techniques throughout this work. Then, we describe a new technique to represent piecewise linear (PWL) functions in optimization problems based on mixed-logical inequalities. The proposed technique is best suited for special class of discontinuous functions that cannot be handled by the regular SOS method. Finally, we introduce our main contribution in this thesis in the fourth chapter, that discusses the unit commitment (UC) and economic dispatch (ED) problems. In this chapter, we present a state-space model in discrete time that can be used to solve both the UC and ED, simultaneously. Such models can be useful when model predictive control (MPC) philosophy is considered to make the scheduling and planning of power systems more reliable and adaptive to changes in the demand side. Further, we show by simulations that the proposed model could be more accurate than the commonly-used ones. We believe that the proposed model can be the core model for all kinds of power systems, not only the isolated one. In the second part, we present our results regarding the control of the generating set (Genset) that comprises a Diesel engine and a synchronous generator. In chapter 5, we present a control-oriented model of the Genset, and design a controller by feedback linearisation to regulate the shaft speed and the terminal voltage, simultaneously, through two control inputs: the fuel mass and the voltage of the field excitation circuit. We provide simulations to show that the proposed controller make the two manipulated control inputs interact with each other, i.e. they both respond to any change of the terminal voltage or the load. In addition, we discuss the robustness of the proposed controller to unmeasured disturbances, uncertainties, and time delays imposed by the Diesel engine, if they are not so large. Chapter 6 discusses special class of marine Gensets or shaft generators. In this type of marine Gensets, the Diesel engine is connected through a clutch and a gear box to a synchronous machine, and the main propeller. Such Gensets can be operated in different modes. Thus, we extend the model proposed in Chapter 5 to include the main propeller. Then, we design a controller to regulate the shaft speed and the terminal voltage in one mode of operation. Also, we provide some simulations to show that the proposed controller can be considered robust to small uncertainties and time delays. The last chapter summarizes the main contributions of this work, and discusses recommendations for possible future work.