Simulation Research Of Flexible DC Transmission Based On New MMC Topology

Modular Multilevel Converter based High Voltage Direct Current(MMC-HVDC)technology is a new type of HVDC transmission technology,which can meet the development requirements of high voltage,long distance and large capacity of HVDC.It is of great significance to the adjustment of power grid structure and the improvement of power quality in China.DC fault handling is a major problem in MMC technology,the widely used Half-bridge Sub-module(HBSM)topology does not have DC fault interruption capability,Full-bridge Sub-module(FBSM)is costly and difficult to control.In the paper,a new MMC sub-module topology with DC fault blocking capability is proposed,which is called Single Diode Clamp Double Sub-module(SDCDSM).The topology,working characteristics and control strategy of SDCDSM are studied,and the feasibility of SDCDSM is proved by simulation.The research contents of the whole thesis are arranged as follows:Firstly,the topology of MMC and the principle of multi-level generation are introduced.Taking HBSM as an example,the working principle of sub-modules is analyzed,and the sub-module topology with DC fault interruption capability is derived.Their output characteristics and fault characteristics are analyzed and compared,and according to analysis,the new topology is designed.Secondly,a MMC sub module SDCDSM with DC fault blocking capability is proposed.The topology structure and working characteristics of SDCDSM are analyzed and studied,and it is compared to the existing sub-module topology in the number of devices used,multi-level output capacity,device voltage level requirements,and the advantages and disadvantages of SDCDSM are summarized.Thirdly,according to the characteristics of SDCDSM,appropriate control strategies are selected,including the nearest level approximation modulation strategy,the loop current suppression strategy,the traditional capacitor voltage sorting algorithm.The principle of these control strategies and algorithms are introduced,and corresponding controllers are designed,and the traditional capacitor voltage sorting algorithm is improved to make it suitable for SDCDSM.Finally,the selection method of capacitor and inductance is introduced.The HVDC simulation model is built in PSCAD/EMTDC,and the steady state,power regulation and DC transient fault of SDCDSM-MMC are simulated.The results prove the feasibility of SDCDSM and the effectiveness of the control strategy adopted.