Research On Topology Of Modular Multilevel Converter With DC Fault Ride Through Capability

Modular multilevel converter(MMC)has the characteristics of no commutation failure,less harmonic content,flexible control and so on.It has become a research and application hotspot in the field of DC transmission and distribution.MMC transmission system based on half bridge submodule(HBSM)has been applied in practical engineering,but it has poor ability to resist DC fault disturbance and cannot block DC short-circuit current,so it is necessary to use cable as transmission line or equipped with DC circuit breaker which can quickly cut off large fault current.In medium and high voltage power transmission and distribution system,the cost of cable is much higher than that of overhead line.Considering the economy,overhead line is the inevitable choice of flexible direct system engineering,so the problem of DC short circuit fault can not be avoided.In this paper,based on the comprehensive study of the existing MMC topology principle and the derived relationship between the topologies,a new MMC sub module topology with anti DC fault interference ability is constructed,and its characteristics and fault ride through performance are studied.The main work of this paper is as follows:(1)On the basis of analyzing the topology and working principle of MMC,the operation principle of typical sub modules is studied,and the transient process of MMC DC short circuit fault based on half bridge sub module and full bridge sub module is analyzed,and the proportion configuration requirements of full bridge sub module which can deal with DC short circuit fault are analyzed in detail.(2)In order to clarify the structural characteristics and evolution rules of sub modules,the sub module topology is classified according to the structural and functional characteristics.The performance of different sub module topologies is compared,and the characteristics of the optimal sub module topology with DC fault ride through capability are analyzed.On the basis of studying the structural characteristics and evolution rules of existing submodules,the essential characteristics of submodule structure are analyzed,and a general submodule topology analysis method is proposed,which provides an idea for the research of new submodules with fault crossing capability.(3)In view of the short board problem of most existing sub modules with DC fault ride through in terms of performance,based on the research of general sub module analysis method,a shifted dual full bridge sub module SCDSM with DC fault current clearing and self-balancing capability is proposed.The characteristics of SCDSM sub module are analyzed in detail,and compared with other sub modules,the advantages of SCDSM in withstand voltage,cost and loss are illustrated.(4)Based on the characteristics of SCDSM reverse input module capacitance and output negative level,the blocking and unblocking DC fault ride through strategies of SCDSM-MMC are proposed,which can effectively block the fault current under serious fault and ensure the smooth operation of the system under no serious fault.The effectiveness of the topology is verified by MATLAB/Simulink simulation system.