Abstract:
Unified power flow controller (UPFC) has the high speed control capability that allows the transmission loading to be increased and to be quickly reallocated to alternative circuits following a disturbance. This increased utilisation of the existing facilities offers an alternative solution to transmission expansion. With increased utilisation, transmission margins are reduced and must be secured against in stabilities. The high speed capability of the UPFC canals to be exploited to provide additional damping of oscillations and to enhance the system transient stability. In this thesis, an investigation on the applications of UPFCs to improve the dynamic and transient stability of power systems is undertaken. The operating principle of the UPFC is explained. Models for representing the steady-state as well as nonlinear and linear dynamic behaviour of the UPFC are developed. These models are incorporated in to multimachine power system models for stability analysis and simulations. A computer program for stability studies is developed for evaluating the performance of the UPFC controllers in enhancing the stability of multimachine power systems. A linearised state model of multimachine power systems is derived for use in the design of UPFC controllers to damp small-signal oscillations. In the design of UPFC controllers, output feedback with constant gains is used instead of state feedback in order to reduce the number of measurements and to enable the use of locally available signals. The analysis of residues is used in selecting the candidate output signals for feedback. A successive pole-shifting method based on eigenvalue sensitivities is developed to determine the feedback gains for the damping controllers.
Simulation results of case studies demonstrating the applicability of the design methodology are presented. To be able to formulate a control strategy for the UPFC to improve the power system transient stability, the effects of the three control parameters of the UPFC: in-phase voltage, quadrature voltage and shunt compensation, on transient stability must be un¬derstood. The utilisation of voltampere capability of the UPFC by each control parameter must also be considered.
Analysis of a single machine-infinite bus system is performed in order to achieve an in-¬depth understanding of the mechanisms of the three control parameters of the UPFC : in-phase voltage, quadrature voltage and shunt compensation, in improving the transient stability. The utilisation of voltampere capability of the UPFC by each control parameter is also considered. Analysis using equal-area criterion reveals that the in-phase voltage and quadrature voltage are very effective in reducing the transient swing. The effective¬ness of shunt compensation is smaller than that of the other methods. Analysis using transient energy function shows that the in-phase and the quadrature voltage controls are particularly effective in increasing the critical energy. The simulation results agrees with the analytical results and show that the quadrature voltage control causes a significantly smaller real power flow through the excitation converter than in-phase voltage control thus leaving a larger voltampere rating of the excitation converter available for shunt compensation. The results of the analysis are very useful in co-ordinating the UPFC control parameters to achieve the maximum effectiveness in improving the power system transient stability. The co-ordination of UPFC control parameters using analytical techniques is very difficult and requires complex on-line computations. This is due to the interaction of the UPFC control parameters through the internal DC power flow and the nonlinear nature of the UPFC. Fuzzy logic does not require precise numerical values of control inputs and system parameters. It allows the knowledge from experiences to be incorporated into the control scheme. Therefore, fuzzy logic control is an ideal candidate for controlling the UPFC. Utilising the results of the above analysis, a fuzzy logic controller for the UPFC for transient stability enhancement is proposed. The fuzzy rules for the controller are designed to minimise and rapidly damp out the transient swings, and to maximise the transient stability margin of the system, without the need for complex on-line calculations. The control strategies make use of the minimum and maximum output characteristics of the generator as affected by the UPFC control parameters. Therefore, the transient fuzzy logic controller can also be applied to other flexible AC transmission systems which have similar effects on the generator output to enhance the power system transient stability. Simulation results demonstrates that the fuzzy logic controller provides a system performance that meets the design objectives and that the controller is robust. The simplicity of the design is the most attractive feature of the fuzzy logic control scheme.