DC Fault Analysis And Overcurrent Blocking Method For MMC-HVDC System Northeast Electric Power University

High-voltage direct current(HVDC)based on modular multilevel converter(MMC)is a new type of transmission technology.Its advantages are mainly manifested in the possibility of low harmonic distortion,low switching loss and high degree of modularity.However,for the traditional half-bridge MMC structure,when a fault occurs on the DC side,the model does not have the ability to block the DC fault current,which will cause the system to greatly reduce the operational safety and stability.Therefore,how to efficiently deal with the MMC-HVDC DC side fault has become a key issue that needs to be solved urgently in the application of DC power grid engineering.Improving the topology of the traditional half-bridge MMC is an effective method to achieve DC fault blocking and clearing.Simply start with the basic topology of MMC and delve into its basic working principles,which in order to achieve MMC-HVDC Topology improvement and DC side fault current blocking provide important theoretical support.Separately set the unipolar ground fault and bipolar short circuit fault on the DC side of the MMC-HVDC system to derive the expressions of the fault current before and after MMC blocking and after the AC circuit breaker is opened under different faults.The correctness of the deduced theoretical fault current model is verifiedthrough the double-end system model based on RT-LAB.which has made a pavement for the further improvement of the system to block the fault current.In order to solve the practical problem that the traditional MMC system can not contain the fault current,this thesis improves its bridge arm transfer branch based on the Current-transferring MMC(CT-MMC)and proposed a DC fault Active-grounded MMC(AG-MMC)topology with clearing capability.When a bipolar short circuit occurs on the DC side of the MMC,the AG-MMC can effectively isolate the circuit breaker and the DC line through its current-breaking branch,and then consume the fault current through the current transfer branch and the energy absorption branchto achieve rapid fault removal.Compared with the traditional MMC structure,the increased device cost and the on-state lossof the proposed AG-MMC topology is relatively low which has strong competitiveness in terms of economy and practicability.Finally,the economic and practicality of the proposed Active Grounded MMC was comparatively verified in terms of building a two-terminal and four-terminal MMC simulation model with the RT-LAB OP5607 software.