Research On Control Method For DC Transmission System Based On MMC

Flexible system has many advantages compared with the traditional DC transmission system, especially the flexible HVDC system based on MMC is closely watched. Multiterminal HVDC system can be more power and more electricity, it make multiterminal system which has obvious advantages more flexible than the double end system, it is widely used in solar energy, wind power and other places. Conventional MTDC system uses Grid phase inverter, however, LCC must have certain network support to the size of the reversing. It must also use a large number of reactive powers. So it must be equipped with reactive power compensation device. In power grid volatility, it will produce commutation failure. It doesn't also supply power to passive system. It also limits the advantage of MTDC.In the thesis, the basic structure of MMC-MTDC HVDC system is analyzed. And the mathematical of MMC-MTDC is also deducted. In the thesis, modulation strategies which are suitable for high voltage dc transmissio n system are compared. The NLC modulation strategy which is suitable for MMC of inverter is chosen. Because there is reactance in MMC arm, on the basis of not ignore bridge arm reactance, steady-state mathematical model of 3-end MMC-MTDC was deduced strictly. On the basis of mathematical model, the coordinated control strategy of the system level is studied. The converter station MMC1 is constant dc voltage converter station, the other two converter station are set as active power converter station. The improve voltage deviation control strategy is proposed in this thesis, considering the dc transmission line resistance of the dc transmission system. An improved voltage deviation controller is designed.When AC system failure occurs, the system will produce negative sequence voltage, it will directly affect the stability of the system. In this thesis, the suppression system controller of negative sequence curre nt and active power fluctuation are designed. On the basis, the principle of negative sequence curr ent control strategy based on voltage compensation is proposed. A simulation model of modular multilevel converter(MMC) based MTDC system in PSCAD/EMTDC is established to verify the correctness and validity of the proposed control method.