Study On Coordinated Control Strategies In AC/DC Systems

The control system is the core of high voltage direct current (HVDC) transmission system. There are some problems have been highlighted in electrical and control fields, such as how to improve the performance of HVDC and how to improve the stability of hybrid system through using the unique advantage of HVDC control and how to cooperate the control components in the whole system. With the gradual implementation of power transmission from the West to the East and nation-wide power grid interconnection,a national complicated AC/DC power grid will be formed in the future. So it is more and more important to study such problems.The thesis focuses on the coordinated control strategies in AC/DC systems, some control methods are presented and the effectiveness is analyzed by numerical simulation. In addition, the tap changer auxiliary controllers for converter transformers are preliminarily studied. The main works are organized as follows:A nine-order dynamic model is established for a single-machine infinite bus system with parallel AC/DC transmission lines, in which the dynamic regulating process of the generator, excitation controller and HVDC are considered comprehensively. The auxiliary modulating signals of the current controller at the rectifier and the extinction angle controller at the inverter, and the reference voltage of exciter are chosen to be control variables. Then the all-state feedback linear optimal controller is designed. The digital simulation results show that the controller has good dynamic performance and can improve the system transient stability effectively.A six-order dynamic model is established for a single-machine infinite bus system with parallel AC/DC transmission lines, in which the dynamic regulating process of the generator and HVDC are considered comprehensively, and the excitation controller is neglected. The modulating signals of firing delay angle at the rectifier and firing lead angle at the inverter, and the excitation voltage are chosen to be control variables. The offset of the current of DC line and the extinction angle and the power angle of generator are chosen to be control outputs. Then a nonlinear controller is designed for the system under study according to direct feedback linearization theory. The digital simulation results show that the controller has good dynamic performance and can enhance the system transient stability obviously.Based on the detailed circuit simulation model of HVDC, the tap changer auxiliary...