Research On Control Strategy And Simulation Verification Of ±500kV Overhead Line DC Grid

Compared with the traditional DC transmission system, Multi-Terminal Direct Current system can be used to realize multi point power transmission, and has more flexible operation mode and economic performance. And the modular multilevel converter (MMC) as a new type of voltage source converter with modular design, is widely used in flexible DC power transmission system in recent years. Combined their advantages, building flexible DC power system based on MMC to achieve renewable energy large-scale access to power grid, interconnection of large area power grid and power supply of city center, has become the main trend of flexible HVDC project construction. The research on control strategy of the flexible DC power grid has the significance in theoretical guidance for the practical application of engineering. Therefore, the main research contents of this thesis are as follows:(1) First, the mathematical model of the modular multilevel converter in the DQ synchronous rotating coordinate system is established, and the control strategy of the flexible HVDC transmission system is summarized.(2) The inner loop current control and outer loop voltage controller are designed, and the PQ operating range is calculated under the condition of weak system. The stability limit of the system under direct current control is studied, and the state variable of the influence factors on the stability of the system is obtained based on the small signal model. Then the power synchronous control strategy is improved, and the stable operation limit of the small signal is obtained. Based on PSCAD/EMTDC the four end flexible HVDC system simulation model is established, the applicability of synchronous power control and direct current control to the weak system are analyzed respectively.(3) Based on the demonstration project of Zhangbei DC power grid, the operation mode of each station under the system level control is proposed;Converter charging control strategy is studied, analysis of the application of DC circuit breaker to remote converter charging control is practicable. Aimed at DC line fault and the converter fault of MMC, the post fault control is researched.