Topology Design Of Modular Multilevel Converters Self-Blocking Sub Modules

Modular Multilevel Converter(MMC)is a new topology used in flexible DC transmission.It has obvious technological advantages in improving voltage level and transmission power,reducing operating loss and reducing output voltage harmonics.How to suppress DC line fault is an unavoidable problem faced by Modular Multilevel Converter basd High Voltage Direct Current transmission(MMC-HVDC).According to the difference of fault current cut-off position,the paper first analyzes three possible fault suppression technical schemes based on AC breaker,DC breaker and MMC sub-module topology and their respective characteristics,then summarizes and compares self-blocking sub-module topology and system-level topology from fault suppression mechanism,control complexity,operation loss,etc.From the perspective of cutting off the fault current,the paper uses the bidirectional controllable switch to improve the design of the half bridge topology,and then combines it with the traditional half bridge to form a hybrid dual sub-module(HDSM).Aiming at the modular multilevel converter composed of this topology,the paper analyzes its fault current characteristics and DC fault suppression mechanism from the MMC equivalent circuit before and after fault lockout,and from the perspective of investment cost and operation loss.The topology is compared to the economics of other sub-module topologies.The analysis results show that the proposed topology can block the DC fault current and reduce the unit level cost and operation loss compared with other self-resistance sub-module topologies.For the MMC-HVDC system composed of HDSM,the paper uses the negative level output characteristics of the designed topology to design the control strategy of HDSM-MMC in fault mode,so as to achieve non-blocking suppression of DC fault.In addition,for the capacitor voltage imbalance caused by the unbalanced charging of the sub-module during the blocking and self-starting period of the proposed topology,the paper proposes a capacitor voltage equalization method by string-connecting the balancing resistor in the charging loop after latching,and designs a four-phase charging strategy of the MMC system to achieve self-excited startup of the system.Finally,the paper establishes a two-end simulation model in PSCAD/EMTDC,and simulates the DC fault suppression characteristics and the effectiveness of related control strategies.