Abstract:
The current world wide trend in power transmission, is to increase the transmission to the maximum limit permitted by the thermal limits of the existing power transmission lines. Power system stabilizers (PSSs) have been used to increase the power transmission limits by improving the dynamic stability of power systems. In recent years, with the advancement of computer technology, a considerable enthusiasm has been directed towards the development and application of controllers based on nonlinear control methods. Power System Stabilizers (PSSs) based on nonlinear control techniques have gained more recognition for power system control than those based on linear control techniques, due to the nonlinear operating characteristics of power systems. Variable structure control (VSC) is one of the nonlinear control techniques which can be used to deal with uncertain and nonlinear control systems. It is the intention of this thesis to investigate the use of variable structure control theory for designing nonlinear robust controllers for power system stabilization. In this thesis, a review of power system stability is presented initially. A linear model of a simple power system described by measurable state variables is reviewed. The possible equilibrium regions of the synchronous machine, subjected to voltage stability and angle stability conditions are derived and the effect of linearizing the nonlinear machine equations around any operating point in this region is discussed. Effect of the excitation system on synchronous machine dynamic stability is then analysed, emphasising the need for Power System Stabilizers (PSSs). A case study describing a conventional PSS design for the example multi machine power system ( under study in this thesis) which exhibits multi modal oscillations is presented. The criterion for selecting the stabilizer function parameters which adequately contribute to the damping of both local and interarea modes of oscillation is explained. Fundamental concepts and features of VSC are briefly outlined. The key steps of VSC design are described with appropriate mathematical derivations. A generalized approach which can be utilized to apply variable structure control theory, to design nonlinear robust controllers is investigated. Application of VSC theory to both nonlinear tracking and regulating control problems is examined comprehensively. The problem of chattering encountered in the proposed variable structure controller is addressed and a provision for eliminating chattering is also presented, which is to be incorporated in the controller design. Use of a linear system model in VSC design so as to guarantee controller robustness is also investigated as a special case. A method whereby a linear system model could be utilized to design a variable structure controller guaranteeing robust performance over the entire possible operating region is proposed. A design based on a linear model of power systems is first employed to evaluate whether variable structure control technique is a viable and superior option for the design of power system stabilizers. A novel approach of incorporating nonlinearities in the system operation at the design stage is presented and the possibility of achieving a robust ·design using a simple linear model of power systems is identified. Effectiveness and performance of the proposed control scheme both in a simple power system as well as in the multimachine power system under study is assessed. The ability of the VSC to maintain system stability, subject to a number of different disturbances and at a number of operating conditions is examined. Design of variable structure power system stabilizers based on a nonlinear power system model is then carried out. Performance and effectiveness of the proposed control scheme in a simple power system as well as in the multi-machine test system is evaluated. Practical issues encountered in the design and implementation of this nonlinear control scheme, such as elimination of controller chattering, elimination of steady state offset that could prevail in the controller output, selection of controller parameters are addressed. Details of implementing the controller using only the measurements local to the controller location are given and suitability of the proposed decentralized control scheme for a multi machine system is asserted. Results of simulation studies demonstrating the ability of the variable structure controllers to co-operate with the conventional power system stabilizers are also presented in this thesis. Further, limitations and related issues of applying variable structure power system stabilizers are addressed, proposing future extensions to the research described in the thesis.