Power system stability improvement using thyristor controlled series compensators

Abstract

Thysistor Controlled Series Compensator (TCSC) is a novel device used in power systems. One of the important functions of the TCSC is to improve the small signal stability of a power system. The aim of this thesis is to investigate the .ability of the TCSC in enhancement of power system stability and to develop techniques for the design of power system damping controllers for TCSCs. In this thesis, a transient model of the TCSC is established by investigating the TCSC dynamics. It is shown that this model is more general and accurate than the existing models for simulating the TCSC transient behaviour. By analysing this transient model, a method for optimizing the transient response of the TCSC is suggested and a suitable TCSC model for the stability studies is identified. The linearization methods available in literature for the conventional power systems is not appropriate for lineraizing a power system with TCSCs. In this thesis, a general method for the linearization of a power system is developed. This method can be used to derive the linear model for a power system with several TCSCs. It is shown in this thesis that the TCSC installation locations are critical for the success of a TCSC damping control scheme in a meshed multi-machine power system. In cases where several TCSC devices are used, the coordination of the TCSC controllers is important for-obtaining optimal performance. Two techniques are developed in this thesis for selecting the most effective TCSC installation location and determining the coordination among TCSC controllers. One is based on analysis of mode controllability. The other is based on analysis of residues. Some important aspects for the design and implementation of the TCSC control schemes are addressed in this thesis. They are (a) the feasibility of designing a control scheme based on a linearized model and its robustness over a range of operating conditions, (b) the calculation of the unmeasurable states and (c) the design of state observers for state reconstruction at the TCSC locations. It is found that the control scheme designed by using a linear model would be robust over a wide range of operating conditions, and the unmeasurable states can be calculated by using a variable transformation. However, the performance of controller-state observer combination is not satisfactory. The reason for this poor performance is discussed in the thesis. One of the most important tasks in the design of decentralised output feedback controllers is the selection of suitable control signals. This aspect is studied in this thesis. It is shown that the existing method for the selection of the output feedback signals has limitations and may provide misleading results. More rigorous methods are developed in this thesis in order. to overcome the limitations of the existing method. In the design of power system damping control schemes, many criteria must be addressed. They include the damping of the critical modes, the robustness over a range of operating conditions, the interaction among the controllers and the capacity limits of the controllable devices. In the conventional controller design method, these criteria are considered in different stages of the design process. Therefore by using the conventional method, it is difficult to optimise the controller parameters. In this thesis, novel methods are developed where the determination of the controller parameters is converted into an optimisation problem. The linear programming and the simulated annealing algorithm are used to find the solutions. By using the proposed methods, all the design criteria can be collectively considered and the optimal parameters for all of the controllers in the control scheme can be simultaneously determined.