| VSC-HVDC(Voltage source converter based high voltage direct current)is a new kind of flexible transmission mode,the basic unit is a voltage source type inverter,its purpose is to solve the problem such as large-scale clean energy grid provides methods,at the same time for the construction of global energy Internet provides a theoretical and practical basis.Firstly,in this paper,a flexible DC transmission system based on Modular Multilevel Converter(MMC)is used as the research object(MMC-HVDC).The topology of modular multilevel converter and the operation principle of the sub module(Sub-module,SM)are introduced in detail.The sub module topology is compared and analyzed for the self-scavenging ability for the principle of construction and operation.Secondly,the starting and running characteristics of the DC side of the MMC-HVDC system,the control strategy of the controlled charging stage,the lightning strike of the transmission line,and the time and condition of the locking are studied.The MMC mathematical model is established.The pre-charge characteristics of the DC side band converter and the impedance frequency characteristic of the DC side are analyzed.On this basis,the simulation parameters of the current limiting resistance are designed.Based on MATLAB,a 21 level MMC-HVDC system simulation model is built,and the simulation results verify the effectiveness of the two stage pre-charge strategy.At the same time,the influence of lightning DC line on DC side fault is studied,and the lightning test of overhead line is carried out,and the insulator state of DC line is evaluated.The fault characteristics of the DC side of the converter are studied.Through the analysis of the fault current characteristics before the failure and the fault current characteristics after the failure and the closure,the MMC locking time is estimated after the fault of the DC side,and the 21 level MMC-HVDC single terminal simulation model built in MATLAB is used to simulate and analyze the time change curve.And taking the MMC-HVDC DC side monopole fault as the research object,the fault current characteristics are compared and analyzed by two methods of clearing the AC side switch and using the full bridge sub-module self-cleaning fault.Based on the MMC-HVDC system,the fault isolation time of the two methods after the DC side failure is estimated.Finally,on the basis of the study of the fault self-clearance mechanism of all bridge sub-modules,the equivalent circuits of all bridge sub-modules in different states are established,and the 21 level MMC-HVDC double terminal simulation model built in MATLAB is used to determine whether the self-cleaning strategy is adopted in the case of the bipolar short-circuit fault occurring on the DC side.According to the simulation results,according to the simulation results,compared with the traditional half bridge sub module using the control and protection of a single AC circuit breaker,the overall control protection based on the full bridge sub-module has a faster response and lower loss in the station when facing the DC side fault.This paper has a certain theoretical and practical value for the reasonable start and stable operation of the flexible DC transmission and the protection of the exchange station in time after the failure. |
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