| With the gradually exhaustion of fossil fuel, the development of the renewable energy such as wind power and photovoltaic power will become very important. However, since the wind power has the features of randomness and fluctation, the ac transmission way is gradually not enough for the renewable energy integretion any longer. In the field of wind power interagation, local and asynchronous power grid connection, the high voltage direct current (HVDC) transmission technology has already shown its advantages. In recent years, the DC grid and multi-terminal HVDC (MTDC) technology has already attracted the focus of industry and academe. Howecer, there is no exactly definiation of DC grid domestic and overseas, and the research of DC grid topology is still at its first stage. Still, it could be for-casted that the future DC grid will apply different topology and voltage level to connect multiple local grids, and to be developed into a main grid to solve the problem arised by wind power.However, since the technology of DC circuit breaker is still not mature, there are many challenges faced by DC grid:1) the DC-DC converters are needed to connect different voltage level transimission lines together; 2) if there is any DC fault in the grid, the DC voltage will be collapsed, and contributes the itterruption of the power transmission. Meanwhile, since the voltage source converter does not have the capability of DC fault iso-lation, the ac system which connected with the DC grid will also be influenced by the DC fatult. Besides, the wind power integeration will also bring large challenges to the DC grid: 1) wind power fluctuation will bring power unbalance to the DC grid; 2) the high penetra-tion wind power will bring the weakness in system inertia and frequency stability.Based on the application of future DC grid and above challenges, this paper researched the modeling and control of a DC system incorporation wind farm and DC-DC converters. The paper first researched the modeling and stability of DC-DC converter which form the DC grid, then design the power balanced and fault isoloation controller of the DC-DC con-verter. After that, the method of coordination control of the wind power integeration was designed. Finally, the model and control of the whole DC grid with wind power system was established and researched.The structure of the whole dissertation is listed as follows:In the first part, the paper summarized the research statues of HVDC technology, in-clude the application of wind power in DC grid and the key technology of DC-DC conveter. The development tendency of future DC grid is summarized, the background and the key problem is anlysised.In the second part, the paper proposed a front to front (MF2F) DC-DC converter. This converter could connect the DC transmission lines of different voltage level, and each DC line could control its power alone. The small signal stability and power unbalance stability of MF2F converter is researched. The conclusion is summarized to lay the foundation of the following controller design.In the third part, the paper proposed an automatical power balanced control strategy for MTDC system. The strategy could keep the power balance without manual control and is applied on a 4-port MTDC with MF2F converter. The simuations of power stepping, ac faults, wind power fluaction has been applied to show the effectiveness of the strategy. The proposed stragety could make MF2F converter applied in DC grid.In the forth part, this paper applied a self-blocked sub-module (SBSM) in MF2F con-verter. The MF2F converter could be used to isolate the fault part in the grid and block the DC fault by itself. The research results show that MF2F not only could increase the stability and reliabity of the DC grid, but also could ensure the faulted lines not to influence on the other part.In the fifth part, the paper proposed a strategy of optimal wind power integration method applied in ac area of DC grid. Focused on increasing the ac power grid stability, a coordinated control is designed with only a small scale of energy storage. This effectiveness of the strategy is validated on simulation tools.In the sixth part, the paper researched the DC grid topology with wind farm integera-tion. The respected start-process, power flow dispatching, DC fault isoloation are re-searched. Modeling and simutions had been done by PSCAD/EMTDC, the simuaiton re-sults shown the control strategy could increase the reliability and flexibility of DC grid.Through this paper, the study results wish to provide references to DC grid and DC-DC converters'development and engineering applications. |
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