| Because of the unmatched advantages, such as no commutation failure, passive network power supply capacity, independent control ability of active and reactive power, large number of output voltage level (low harmonic content), suitable for constituting of multi-terminal HVDC transmission, MMC-HVDC(Modular Multilevel Converter) becomes a hotspot for the research and application in HVDC system, which is widely used in the fields like connection of asynchronous system, grid connection of new energy, offshore drilling platform, islanded power supply, capacity expansion and reformation of urban distribution network and so on.Firstly, the mathematical model is built by analyzing MMC work principle, inner and outer loop control strategy is proposed and the effectiveness of the basic control strategy is verified by simulation. Secondly, by building circulation mathematical model under the two-phase static frame, circulating current restraining control based on quasi proportional resonant (PR) is designed, which greatly reduces parameter adjustment and restrains 2nd harmonic in arm-currents effectively, and reduces distortion of arm-currents and the voltage fluctuation of sub-modular(SM). Thirdly, start-up control strategies under two conditions are researched:when connected with passive network or active grid to be started, DC charge method is applied, which needs on auxiliary power supply and limits voltage and current surges during charging; when outage MMC station parallels in DC grid, a new grid-connected control is used to realize grid connection by selecting which time to close DC switch, thus improving stability and reliability of power system. Finally, positive and negative mathematical models of MMC are built by symmetrical components method. Negative current restraining control is added in converter controller to restrain the negative components. The feasibility and validity of negative restraining control strategy to deal with unsymmetrical faults are verified by simulation results. |
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