Key technologies of three-terminal DC transmission system based on modular multilevel converter

A three-terminal DC transmission system based on modular multilevel converter (MMC) is developed to solve the problem of power supply for distributed islands or cities. The system, with flexible features, may generate high quality output voltage and reduce construction costs and power losses in the converter stations. However, the three-terminal DC system based on MMC faces more complex situations in operation and its control strategies. In this paper, the mathematic model of three-terminal DC system based on MMC is deduced. Then according to the structure of MMC, a method of carrier phase-shift SPWM is introduced. As the system-level control, the decoupling controller for three-terminal DC system based on MMC is designed. A multi-point DC voltage control strategy based on DC voltage error method, which doesn´t need communication between converter stations, is proposed to improve the system reliability. By simulation with PSCAD/EMTDC, the feasibility and effectiveness of the proposed model and control strategies are verified.