| Compared with the land wind, offshore wind power has the advantages of high wind speed and low eddy current loss, developing offshore wind is a new trend of the development of wind power. But generally speaking, offshore wind farm is far away from the load center, and the wind is intermittent, uncontrollable, therefore, the research on grid aceess technology is an important topic.This paper begins with the working principle of VSCF wind power generation system which currently occupies the mainstream status, then compares several offshore wind power grid access mode from economic, transmission capacity and the dynamic or static characteristics of the system, the results show that the flexible high voltage direct current transmission based on modular multilevel convert which called MMC-HVDC system is very suitable for grid access of offshore wind power which has long distance and large capacity. This paper is aiming at some further research on the application of MMC-HVDC in grid access. The exact contents are listed as follow.1) This paper analysed the working principle of MMC, studied the modulation method.2) This paper proposed a seed module capacitor voltage balancing control strategy suited for MMC, which is combined with carrier phase shift modulation strategy (carrier phase-shifted SPWM, CPS-SPWM), and based on the method of weighting balance signals on reference. The voltage-balancing control can be divided into averaging control and balancing control. Simulation results testified the validity of this method.3) The mathematic model for MMC-HVDC is established in (a,b,c) and (d,q,o) coordinate system. At the same time, we studied the control method of MMC-HVDC system. Moreover, the control strategy of MMC-HVDC system connected to passive network and MMC-HVDC system connected to active network is carefully studied. Using simulation software Matlab/Simulink, we built the simulation model of MMC-HVDC system connected to passive network and MMC-HVDC system connected to active network.Simulation results prove the advantage of system and the correctness of the control method.4) According to the DC bus type grid access of offshore wind power, this paper proposed a multi input multi output topology. Using simulation software Matlab/Simulink, we builted the simulation model. The simulation results show that, the designed system has good disturbance rejection properties and schedulability is strong. |
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