| With its outstanding advantages of low harmonic content,low switching losses and flexible voltage level,etc.,Modular Multilevel Converter(MMC)has received widespread attention in the field of high voltage direct current(HVDC).The impedance of DC system is smaller and the speed of fault diffusion is faster,thus DC fault protection has been one of the major problems that MMC-HVDC faces in industrial popularization and is also the focus of this paper.Firstly,the topology structure and working principle of MMC are analyzed,and the control system of MMC-HVDC transmission system is described.Secondly,the fault characteristics of DC pole-to-pole short-circuit fault and pole-to-ground fault are analyzed.The analytical formulas of fault current are deduced,and the simulation results verify the good accuracy of the formulas.The fault characteristics and equivalent circuits of DC line breakage fault are explored.Thirdly,some typical topologies of the hard blocking technology and soft blocking technology are quantitatively analyzed from the aspects of reliability,cost and DC fault blocking performance,thus a comprehensive evaluation index is proposed to quantify the different topologies with DC fault blocking capability.Finally,in view of the overcurrent limiting before the converter is blocked,the principle of MMC with coupled arm inductors is studied.The influences of coupling coefficient on the normal operation and fault characteristics of the system are studied,and the good performance of current-limitting is verified by simulations.Aiming at the overcurrent limiting after the converter is blocked,the MMC with current-limiting resistors and novel submodules is proposed,and its fault current limiting mechanism is analyzed.By analyzing the fault blocking time and the electrical stress of the devices,the selection criteria of key parameters are presented.The simulation results show great current-limitting capability. |
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