Decentralized Robust Load-frequency Control Synthesis in Restructured Power Systems

This dissertation is mainly focused on technical issues associated with load-frequency control (LFC) in restructured power systems. Operating the power system in the deregulated environment will certainly be more complex than in the past, due to the considerable degree of interconnection, and to the presence of technical constraints to be considered together with the traditional requirements of system reliability. However at present, the power system utilities participate in LFC task with simple, heuristically tuned controllers. In response to the new technical control demands, the main goal of this dissertation is to develop the robust decentralized LFC synthesis methodologies for multi-area power systems based on the fundamental LFC concepts and generalized well-tested traditional LFC scheme to meet the specified LFC objectives. The dissertation is organized as follows: Chapter 1 gives a general introduction on load-frequency control problem and its conventional control scheme. The past achievements in the LFC literature are briefly reviewed, and the main objectives of the present dissertation are summarized. Chapter 2 introduces modified models to adapt well-tested classical LFC scheme to the changing environment of power system operation under deregulation. The main advantage of the given strategies is the use of basic concepts in the traditional framework, and avoiding the use of impractical or untested LFC models. The introduced structures provide the base models for robust LFC synthesis in the subsequent chapters. Chapter 3 presents two robust decentralized control design methodologies for LFC synthesis using structured singular value theory (µ). The first one describes a new systematic approach to design sequential decentralized load-frequency controllers in multi-area power systems. System uncertainties, practical constraint on control action and desired performance are included in the synthesis procedure. The robust performance in terms of the structured singular value is used as a measure of control performance. The second control methodology addresses a control approach to design of robust load frequency controller in a deregulated environment. In this approach the power system is considered under the pluralistic-based LFC scheme, as a collection of separate control areas. Each control area can buy electric power from some generation companies to supply the area-load. Multi-area power system examples are presented demonstrating the controllers’ synthesis procedures and advantages of proposed strategies. In chapter 4, the decentralized LFC synthesis is formulated as an H∞-based static output feedback (SOF) control problem and is solved using an iterative linear matrix inequalities (ILMI) algorithm to design of robust PI controllers in multi-area power systems. Two multi-area power system examples using both traditional and bilateral based LFC schemes with a wide range of load changes are given to illustrate the proposed approach. Chapter 5 is organized in two main sections. In the first one, the LFC problem is formulated as a multi-objective control problem and the mixed H2/H∞ control technique is used to synthesis the desired robust controllers for LFC system in a multi-area power system. In the second section, with regard to model uncertainties, the multi-objective LFC problem is reformulated via a mixed H2/H∞ control technique and then in order to design a robust PI controller, the control problem is reduced to a static output feedback control synthesis. Finally, the problem is easily solved using a developed ILMI algorithm. The proposed methods are applied to multi-area area power system examples under different LFC schemes. The results are compared with pure control design. Chapter 6 summarizes the research outcomes of this dissertation.