| As a renewable energy source, wind energy has been more and more importantly attached. Modular multilevel converter based HVDC (MMC-HVDC) is an integration of modular design technology, which can eliminate the drawbacks of traditional VSC-HVDC and provide some additional benefits with respect to the performance as well. In this thesis, wind farm is connected with MMC-HVDC, then, MMC-HVDC as the starting power starts power system restoration.Firstly, structure and steady characteristics are analyzed in theory about Double Fed Induction Generator (DFIG), in the theory based on optimal wind power capture principle and stator flux orientation, and combining with the DFIG mathematical model, control strategy can be deduced about DFIG stator. Secondly, this paper introduces the topological structure and work principle of MMC. Based on the equipartition of energy and voltage balance, a voltage-balancing scheme of sub modules is presented in this paper with the topology of MMC and modulation. Then the harmonic problems of MMC-HVDC are analyzed, the fundamental and harmonic characteristic are analyzed under the CPS-SPWM strategy. Thirdly, the paper establishes the MMC-HVDC system model connecting with wind farm, analyses the steady characteristics of MMC-HVDC and designs corresponding control strategy. At last, the problems about power system restoration are analyzed in the paper, no-load line effect power frequency over-voltage formation mechanism is importantly analyzed in Power System Restoration and advantages are introduced by using MMC-HVDC as the starting power in Power System Restoration. Based on the theory above, control strategy has been designed to restrain power frequency over-voltage.Relative simulation models are built in PSCAD/EMTDC to validate that the control strategies proposed by this thesis are feasible and effective. |
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