| Modular multilevel converter (MMC) is a new multilevel converter topology suitable for VSC-HVDC. The modular design is adopted in MMC, and its modulation is multilevel modulation technology. Compared with the conventional VSC-HVDC, it deserves many advantages. In this thesis, the topology, working principle and modulation technology of MMC are analysized. Based on the aforementioned analysis, the steady-state control strategy and transient control strategy of MMC-HVDC are studied.First of all, the topology and working principles of MMC-HVDC are analysezed, and the difference between MMC and conventional two-level VSC in topology are proposed. Through comparing the multi-level converter modulation strategies, the phase shift PWM is applied in MMC-HVDC. Secondly, compared with the conventional two-level VSC, there is bridge reactance in MMC. On the basis of considering the bridge reactance, the mathematical model of MMC-HVDC on steady-state is derived, and the current decouping controllers of fixed DC voltage and fixed reactive power in MMC1and fixed active power and fixed reactive power in MMC2are designed. The21level MMC-HVDC is created in PSCAD/EMTDC, the simulation verifies the mathematical model is correct and the control strategy is effective. Finally, the positive-and negative-sequence mathematical models are established when unbalanced fault occurs in the AC system. For the characteristics of negative-sequence current and fluctuating active power of MMC-HVDC under AC grid faults, the current controllers of eliminating negative-sequence current and fluctuating active power are designed. For the active power transmits unbalanced and the DC voltage can not be controlled while the AC system of the DC side fault, the conversion model of DC side is designed. The aforementioned transient control strategies are simulated in PSCAD/EMTDC, and the simulation results verify the effectiveness of the control strategies. |
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