| Voltage Source Converter-Based HVDC (VSC-HVDC) is a novel DC transmission technology, which is characterized by quick and independt control of active and reactive power, power supply to small isolated long distance passive load, generation of reactive power to AC system as well as the ability to make up easily multi-terminal network. VSC-HVDC technology relates to many academic fields such as electrical technology, modern power electronics technology, and automation control theory, etc. Because VSC-HVDC has emergered recently with a short period of development, many basic theories and application issues need to be investigated deeply, especially the mathmetic model, operation characteristic and mainly application questions.Aiming at the stable and dynamic characteristics of VSC-HVDC, new transmission type conjointed with tranditional HVDC technology as well as its application in power system restoration, the following results are achieved:1. The rectangular coordinate form of VSC's power transmitting equations is deduced through coordinate transformation. By using the nonlinear control of inverse system, VSC-HVDC control system is designed based on that active and reactive power is adjusted respectively, and then the active and reactive power of VSC can be controlled in four quadrants. When three phases voltage are asymmetrical, the transmitted power of VSC fluctuates and the operation of VSC-HVDC is influenced. The power characteristic of VSC under asymmerical voltage is drawn according to instantaneous power theory. By adopting asymmetrical control scheme, the fluctuation of active power caused by asymmerical voltage is eliminated. The configuration, active and reactive power operation characteristics and its influence on AC voltage of VSC-based distribution network are also analyzed.2. The dynamic mathematic model of VSC-HVDC with SPWM control is founded and its switch characteristic and dynamic operation mechanism under changing load and fault condition are analyzed. The foundation of dynamic mathematics model of VSC-HVDC based on energy conservation principle, the analysis of the process of energy conversion and dynamic operation mechanisms, and the deduction of energy banlance relationship, are realized.3. Conventional HVDC has the advantages of mature state of art and lower cost. VSC-HVDC has the advantages of flexible operation and control and the disadvantages of high cost. Combining the advantages of the two kinds of transmissions, a novel hybrid HVDC transmission is proposed. And then its configuration, technology specialty, operation characteristic and mechanism, regulation type and characteristic as well as active and reactive power operation diagram, are discussed.4. The self-protection for VSC-HVDC used in power system network interlinkage when power system has blackout is analyzed. A"soft shut-off"control strategy is put forward, which can make VSC-HVDC quit the deliver of power and then operate as the spinning reserve power source without acute change and equipment breakage. Through discussing the overvoltage restraintion question of no-load long transmission line, the soft-startup of VSC-HVDC in the case of one side AC system has blackout is developed to avoid severe operation overvoltage of switching on the no-load long line. Based on Lyapunov stability theory, control strategy for VSC-HVDC in network restoration is designed, adopting backstepping method to improve the stability of power system restoration.
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