| Flexible DC transmission and distribution system has many advantages, like rapidly and independent control the active and reactive power, stabilize AC voltage and being able to delivery power to the wack or passive network. It can be widely used in asynchronous power grids interconnection, renewable energy grid, improving power quality and deliverying power to inland. With the development of full-controlled power semiconductor devices, not only the cost of installing and operating of flexible DC transmission and distribution system is reduced, its transmission capacity and voltage levels are improving gradually. The flexible DC transmission and distribution technology has greatly expanded the field of application for DC system and attracted more and more home and abroad scholars to engage in the research on it. Right now, the research on flexible DC transmission and distribution system is still at beginning. This dissertation focuses on the system structure, mathematical modeling and control strategy. It includes the following contents:1. The system structure and principle have been analyzed and then the mathematical model of converter system has been deduced. With the help of coordinate transformation matrix, the equivalent circuit of converter system is obtained under different axes. The controller of converter system has been designed based on direct power control (DPC) algorithm. The two degrees of freedom based outer voltage square loop of DPC algorithm has been applied in two terminal flexible DC transmission and distribution system controller design. The mathematical model and controller’s performance has been verifed by simulation.2. The DC voltage and active power fluctuation has been analyzed when the unsymmetrical fault happened in grid. An improved DPC algorithm, which uses trap filter and PI controller as an outer loop and applies instantaneous positive sequence power instead of instantaneous power as an inner loop has been proposed. The simulation results show the new controller can suppress the DC voltage fluctuated and reduce the current harmonic distortion effectively. 3. Based on three-phase to single-phase traction converter system, a new cophase traction power supply system scheme has been proposed. The system structure and principle have been analyzed and the controller of single-phase converter system has been designed by synovial control theory. The scheme has been verified by simulation and prototype test and its result shows that the controller can keep the voltage and frequency stable and the proposed scheme can solve the power quality problems effectively.4. The control strategy of the multiterminal system has been researched. A scheme combined the master-salve DC voltage control strategy and DC voltage droop control strategy is proposed. This scheme can realize automatic switching control modes according to the system’s performance. |
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