Research On Topologies And Control Methods Of Tri-pole HVDC Based On MMC

It is widely know that AC is not the best way to connect wind energy and increase the capacity of the transmission line. With the development of the HVDC, there is a strong need to convert the exiting AC transmission line to HVDC transmission line. Tri-pole HVDC based on MMC is one of the most attractive ways because it can increase the capacity and the efficiency of wind energy integration, making contribution to developing an environment-friendly and land resource-saving power system.Compared with Tri-pole HVDC based on Line commutated converer, tri-pole HVDC based on MMC has the following advantages:no commutation faluire, independent active power and reactive power control, no reactive compensation, small footprint, therefore it is an attractive topology for reconstruction. This thesis will be focused on topology and control method of tri-pole HVDC based on MMC.In the second section, the principle of tri-pole HVDC based on MMC and the factor of affecting adding capacity will be analyzed, then the current modulation is described, after that the topologies of sub-module with DC fault ride-through capacity and voltage reversal are investigated and the corresponding conclusion is presented.The third part focuses on the symmetrical tri-pole HVDC based on MMC. Firstly the modes of operation, the principle of power exchange,the advantage and disadvantage of symmetrical tri-pole HVDC based on MMC are analyzed, then in order to reduce the fluctuation of active power and the AC voltage, the optimized current modulation for overhead line is investigated. After that, the optimized voltage and current modulation for cable is proposed. In order to distribute the reactive power among the three converters, a coordinated method is proposed, and the control method for symmetrical tri-pole HVDC based on MMC is analyzed. After that five-pole and 2n+1-pole HVDC are proposed. Finally a model is established in PSCAD/EMTDC and the feasibility of the symmetrical tri-pole HVDC based on MMC is verified.In order to overcome the disadvantage of the symmetrical tri-pole HVDC based on MMC, some novel structures are investigated in the fourth part. Firstly, the universal topology and the grounding method of the tri-pole HVDC based on MMC with DC power plow controller are analyzed. Then two specific structures of tri-pole HVDC based on MMC are described. Because the DC power flow controller is not mature yet, the asymmetrical tri-pole HVDC based on MMC is proposed and a model is established in PSCAD/EMTDC to verify the control method. Finally different kinds of the tri-pole HVDC based on MMC are compared.