Coordination Control Of VSC-MTDC System And Design And Implementation Of The Simulative Experimental System

With the development of electric power technology, communication, computer and automation technology has been widely applied in power system, at the same time, in order to realize the development, transportation and consumption of the clean energy, the flexibility and compatibility of the grid must be improved. Under this background, smart grid become the inevitable trend development of the grid, the flexible direct current transmission (VSC-HVDC) system is an important part of the smart grid, therefore, the research on the VSC-HVDC system has important theoretical and practical significance.The advantages of the VSC-HVDC technology are large transmission capacity, good controllability, dynamic reactive power compensation, the capability in improving power quality and the environmental friendliness. The VSC-HVDC technology is a grid-connected way of the wind power, which is recognized that it has great advantages in the world. The multi-terminal flexible direct current transmission (VSC-MTDC) system is a VSC-HVDC system with multiple voltage source converters (VSC), therefore, it not only has the characteristics of the two-terminal VSC-HVDC system, but also can be used to solve the transmission problem of multiple power supply or multiple power consumption, the power flow between the converter stations can be coordinated, and the power system can be more economic and flexible. However, compared to the two-terminal VSC-HVDC system, the coordination control of the VSC-MTDC system and the changes of the power flow are more complex, therefore, it is necessary to study on the coordinated control strategy and simulative experimental system of the VSC-MTDC system.Firstly, the mathematical model of the three-phase two-level VSC system is established in this thesis, the model can be decoupled, and it is conducive to the design of the controller; then the hierarchical control principle of the VSC-HVDC system is given, the vector control is adopted in the control of converter station, and the inner loop current controller and outer loop voltage controller are designed respectively, the simulations in PSCAD/EMTDC verify the effectiveness of the control strategy.Then, in view of the deficiency of the existing control strategy of the VSC-MTDC system, the characteristic curve of the fixed active power control is improved in this thesis, based on the traditional DC voltage-active power regulation characteristic curve. The slope control and fixed DC voltage control are added in the characteristic curve, and the hysteresis control is added during the switching moment of the control modes, and then the coordinated control strategy of the VSC-MTDC system based on the improved DC voltage-active power regulation characteristic is proposed. Then the DC voltage calculation of the VSC system is analyzed, and the system is simulated in PSCAD/EMTDC.Next, the hardware and software of the VSC-MTDC simulative experimental system are designed in this thesis. The IPM module is the core of the converter station, the peripheral circuit of the IPM module and the power supply circuits are designed in this thesis; the controllers of the system are DSP chip TMS320F2812and FPGA chip EP3C25Q240C8, each controller controls two converter stations; the main control circuit schematics of the system are given, and the system software consists of the main program, the A/D sampling subroutine, the control interrupt subroutine, and the digital PI algorithm subroutine.Finally, based on the design of the system hardware and software, a VSC-MTDC simulative experimental system, which is a low voltage and low power system, is establish in the thesis, and the system experiment is carried out. From the experiment, we can see that the hardware and software design of the VSC-MTDC simulative experimental system is effective and feasible.