| With the development of power industry, full-controllable electric semiconductor devices are increasingly applied to electric power transmission and distribution, power quality improvement, etc. VSC-HVDC technology based on full-controllable electric semiconductor devices is rapidly developed. VSC-HVDC technology has the advantages such as flexible power control, supplying power to passive network, and forming multi-terminal dc network, etc.Voltage source converter based multi-terminal direct current (VSC-MTDC) technology is to point to those HVDC Flexible system which has three or more VSC in the same dc system. It not only has all the characteristics of two terminal VSC-HVDC system, at the same time can also solve these electricity transmission problems such as multi-terminal power supply and placement. As the VSC has been applied in the field of high voltage direct current transmission widely, as well as its unique advantages in the field of new energy integration and building urban grid.VSC-MTDC transmission system compared to two terminal VSC-HVDC system has higher efficiency and flexibility, more broad application fields, but at the same time, the design of its controller and the choice of control strategy are more complex. So how to design or select the appropriate control strategy is the key to build multi-terminal dc system. At present, the existing research achievements are always staying in digital simulation phase. However, VSC-MTDC system is a complex, nonlinear and multi-variable system. So it is necessary to establish a dynamic simulation experiment platform for multi-terminal dc system experiment study, to reflect their operating characteristics in the actual running environment.Firstly, this paper introduces the research and development status of HVDC flexible technology in the domestic and overseas, analyses the main topology structure and operation principle of VSC, as well as the mathematical model and decoupling control in the different coordinate systems. Then, based on the principle of hierarchical control, combined with vector control, the design of main circuit system is given, and simulation model is set up in the PSACD/EMTDC. The simulation is carried out to verify the effectiveness of the proposed control strategy.After that, aiming at the insufficient of VSC-MTDC in dc fault blocking at present stage, this paper analyzes the different structures of modular multilevel converter (MMC) in the case of short circuit current situation under dc fault, puts forward an improved MMC topology structure, and designs a hybrid MMC topology structure suitable for VSC-MTDC transmission system based on the mismatched-cascade mechanism and the principle of dc fault. Combining the dc fault "handshake" principle of VSC-MTDC transmission system, this paper specifics the way to realize the fault line removal and the fault recovery process.Then, this paper designs the hardware and software for the dynamic simulation of VSC-MTDC transmission system, respectively. Based on PM10CSJ060 Mitsubishi IPM module, with the TMS320F2812 fixed-point type DSP controller as the core, the proportional dynamic simulation model of VSC-MTDC transmission system is established. In this paper, the main circuit, peripheral circuit parameters and selection of the device are analyzed. And the principle diagram of system main circuit is given.Finally, this paper, on the basis of the design, builds a low voltage and low power dynamic simulation experiment platform for VSC-MTDC transmission system, designs a set of wind power and PV power grid-connected inverting devices which are connected to the VSC-MTDC dynamic simulation platform, carries on some typical system experiments, gives the experimental waveform and the result analysis. At last, the validity and feasibility of the VSC-MTDC control strategy is verified by these experiments. |
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