DC Fault Analysis And Overcurrent Suppressing Method For MMC-HVDC System

As a new transmission technology,MMC-HVDC has many advantages such as high modularity,small harmonic distortion,and low switching loss.It has been rapidly developed in the field of HVDC transmission.The typical half-bridge MMC does not have the ability to block the DC fault current,which seriously threatens the safe operation of the system.Therefore,the DC fault handling of MMC-HVDC is one of the key problems to be solved in the application of DC grid engineering.Firstly,the topology and working principle of MMC are analyzed in detail.The fault mechanisms of three kinds of faults on the DC side of MMC-HVDC are analyzed in detail,including pole-to-pole fault,monopolar grounding fault and line breaking fault.The expressions of fault current before converter blocking,after converter blocking and after AC circuit breaker blocking are deduced in the process of pole-to-pole fault.A two-terminal system is built in the simulation platform,and the analysis results of three kinds of faults are verified.These provide a theoretical basis for the reasonable design of DC fault overcurrent suppression methods.Secondly,for over current in converter bridge arms before blocking in case of DC short-circuit faults in MMC-HVDC system,a method based on coupled bridge arm reactors is presented,which can effectively suppress fault current,and improve the steady state operation of the system.The MMC-HVDC system model with coupling bridge arm reactors is established,the coupling principle of reactors is clarified,the fault current is analyzed,and the simulation model is built in the simulation platform.The parameters of system based on coupled bridge arm reactors are compared with that of system based on ordinary reactors in the two phases of normal operation and fault.The result shows that using coupled bridge reactors can significantly reduce fault current.Finally,to solve the problem that half-bridge modular multilevel converter being incapable of blocking dc fault current,a current-transferring MMC(CT-MMC)is proposed.Compared with half-bridge MMC,the CT-MMC added three branches of current breaking branches,energy absorbing branches,and current transferring branches.Through the cooperation of each branch in the CT-MMC,the fault current is cleared.And its topology is established and the fault clearing process is analyzed in detail.Referring to the half-bridge MMC with the same voltage grade,the CT-MMC has a good economy and its additional device cost is low and additional conduction loss is very small.The simulation results verify the effectiveness of the proposed solution.The research in this paper realizes the suppression and elimination of the fault current overcurrent on the DC side of MMC-HVDC,and reduces the damage to the system caused by the fault,which is of great significance for improving the safe operation of the system.