| ABSTRACT:The engineering application of Flexible HVDC technology is important in our country, for improving the security and stability level of China’s power grid and establishing economic, environmental, efficient and advanced intelligent transmission and distribution systems.MMC-MTDC (Multi-Terminal Direct Current based on Modular Multilevel Converter) transmission system possesses technological advantages of MMC high voltage, high-capacity and operating characteristics of MTDC system flexibility, reliability and economy. Therefore, MMC-MTDC has become one of research hotspots in HVDC field. This paper mainly studys coordinated control strategy and the upper control in MMC-MTDC system, and verifies the control strategy by real-time simulation and experimental platform. The main contents include:(1) Converter valve control of MMC-MTDC transmission system is studied. Taking the voltage level and the number of modules into account, carrier phase shift modulation and module voltage control are selected by contrast analysis of different modulation schemes and voltage control. The mathematical model, control system and control designer of MMC-MTDC system are detailedly introduced. The control method and control target of single MMC converter station is focused on.(2) Converter station control of MMC-MTDC transmission system is studied. Three coordinated control strategys in MMC-MTDC system-the master-slave control, DC voltage margin control, DC voltage slope control, are comparative analysis in detail.(3) Two improved control strategies-droop curve panning control and droop curve slope control are proposed based on problems of DC voltage slope control. On this basis, a new DC voltage slope control is proposed combining two improved control, eliminating static bias and avoiding the overvoltage when power changes greatly. Furthermore, the stability analysis is carried out based on the new coordinated control strategy to obtain the relationships of controller parameters and stability.(4) System-level control of MMC-MTDC transmission system is studied. Common upper layer control of MMC-MTDC system and upper layer control of MMC-MTDC system with different local control are detailedly analyzed. For problems of urban distribution network, the new upper layer control is proposed and verified on the background of distribution network in the city.(5) The real time simulation models of MMC-MTDC system are built in RT-LAB real time simulation platform, and the correctness and validity of the proposed control strategy is verified. In addition, by MMC-MTDC system experiment platform, the control strategy and theoretical analysis is verified again. |
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