Investigation On The Control Strategy Of MMC-HVDC Transmission System Connected With Wind Farm

With the depletion of fossil resources, wind energy is gradually being concerned by people. Compared with the onshore wind farm, the offshore wind farm has higher efficiency and lower construction cost, so it is more popular with people. Connecting the offshore wind farms through AC transmission has some disvantages:high active power loss and voltage drop,decline of AC power grid reliability. With the rapid development of power electronics technology, DC power transmission technology has been applied in engineering,and the third generation of DC transmission technology based on Modular multi level converter(MMC)can effectively solve the problems mentioned above.The mathematical model and control strategy of DFIG are analyzed theoretically, the model of DFIG is established by using PSCAD/EMTDC simulation platform and the model can accurately reflect the characteristics of the actual fan. The operation principle and control method of MMC and MMC-HVDC transmission system are analyzed and the double closed-loop control strategy is designed for the MMC-HVDC transmission system. Two-terminal MMC-HVDC transmission system is built by using PSCAD/EMTDC simulation platform and the effectiveness of the control strategy is verified by simulation.The two-terminal MMC-HVDC transmission system connected with wind farm is studied. When the symmetrical faluts occur in AC side,the change of DC voltage is converted to frequency change by the formula to control the rotor speed of DFIG to store energy and the active power of wind farm can be regulated so that it can reduce the fluctuation of the DC voltage during the fault period. When the asymmetric faluts occur in AC side,the proposed control strategy can restrain the negative sequence current and maintain the active power transmission to the grid side during the fault period which reduces the unbalanced power of the system during the fault period. Simulation results show that the control strategy can restrain the DC voltage fluctuation during the fault period and makes the DC system run stably.Three common kinds of control strategies including master-slave control,multi-point voltage drop control and DC power coordination control of multi-terminal MMC-HVDC are described in detail. The advantages and disadvantages are analyzed according to the comparison of simulation results. A coordinated control strategy based on improved voltage drop control is prop osed. This control strategy can switch between different control modes according to the running state of the converter connected with local grid and remote grid and operating range of DC voltage. The four-terminal MMC-HVDC transmisson system connected with wind farm is established by PSCAD/EMTDC simulation platform,and simulation of different degree of wind farm power fluctuation,voltage station exit and three-phase fault of the AC grid is carried out to verify that the proposed control strategy can effectively improve the reliability of the system.