Nonlinear Modal Analysis And Flexible Coordinated Control For HVDC/AC Power Systems

One of the prominent characteristics of modern power systems is to form large-scale interconnected power networks. Due to its distinct advantages, HVDC plays an important role in long distance power transmission and large-scale interconnected power systems. Consequently HVDC has been being adopted in more and more power systems. The interconnection of power systems is to make power generating and power transmitting more economical and reliable, but at the same time it brings many new dynamic problems that influence the operating stability. The primary means to improve power systems' stability is to develop their control systems. Moreover, the reliability of an interconnected power network needs to be guaranteed by developing advanced control system. With new-style devices such as HVDC and FACTS present in power systems, on the one hand the controllable degree of power systems is increased, and on the other hand, the stability and control characteristic of power systems become more complex. Because that each of the controllers uses local and uncoordinated control strategy may cause unstable interactions between them, it is necessary to integrate and coordinate all kinds of controllers. In a word, the development of interconnected power systems makes power system stability and control problems more prominent and important.This dissertation focuses on the stability analysis and the control study for HVDC/AC interconnected power systems, especially the system characteristic analysis and control for HVDC links. In these two fields, some innovative work has been made: (1) The power system nonlinear modal analysis method based on modal series is proposed, and for the first time it has been successfully applied to HVDC/AC power systems. (2) A brand-new concept and frame named Flexible Coordinated Control System for Interconnected Power Networks is proposed. Underthe frame of the Flexible Coordinated Control, this paper presents a multi-agent system based power system flexible coordinated control system and applies it to the coordination and optimization of HVDC modulations in multi-infeed HVDC transmission system.Firstly this paper summarizes the research achievements that overseas researchers made in power stability analysis, especially in modal analysis techniques. Based on this, this paper develops the modal series method, expounds its mathematic foundation and theory system, and compares it with the linear modal analysis and the method of norm form of vector fields. The modal series method is brand-new in its application in power systems, which extends the linear system theory concepts to facilitate the understanding and analysis of nonlinear system.For the first time, modal series has been successfully applied to HVDC/AC power systems. This paper systematically explains the modal series based HVDC/AC power system analysis method, and proposes a comprehensive analytical technique based on modal series theory and symbol operation symbolic computer algebra for the analysis of modal interaction of stressed power systems that include HVDC links. A method for dealing with the AC-DC interface is presented in this paper, the concept of which is very clear. This method is general and may be extended to include other FACTS devices. A power system analysis software package has been developed. By using this program, the nonlinear system response and an approximating closed form expression for the zero-input response of the nonlinear system can be obtained. This program can also be used to analyze the system mode and the nonlinear modal interaction, and provide valuable information for control system design.According to the research objectives, several performance indices evaluating the nonlinear effects and the nonlinear interactions in power system dynamic responses are defined. A three-generator pure AC power system and a HVDC/AC power system are studied as test systems by employing modal series method. In this paper, we study the system dynamic behavior after subjecting to a disturbance, and analyze the system stability, oscilla...