| With the development of power electronics technology,the modular multilevel converter MMC system using fully controlled IGBT devices as switches is becoming more and more widespread in practical engineering applications.DC transmission based on MMC converter(MMC-HVDC)is an emerging transmission technology in recent years.With its advantages of large transmission power,low line cost,and good control performance,it will be used in large-scale power grid interconnection,urban power supply,and new energy.It plays a huge role in the power transmission environment such as grid-connected power generation and island power supply.At present,domestic and foreign research scholars' MMC-HVDC system fault location research mainly focuses on AC side faults and internal faults in MMC converter stations,and there is less research on DC transmission line faults.Therefore,the main content of this thesis is the research and analysis of the fault characteristics and fault location of the MMC-HVDC DC transmission system.This thesis starts with the topology and working principle of the MMC-HVDC system,and makes a corresponding research and analysis of the internal modulation mode of the MMC converter station and the sub-module capacitor voltage equalization control.And the MMC-HVDC DC transmission system model was built on the simulation platform of PSCAD / EMTDC,and the working conditions during its steady-state operation were observed and analyzed,which provided a basis for subsequent failure analysis.Secondly,three different types of faults are set up for the MMC-HVDC DC transmission line,the fault characteristics are analyzed,and the changes of various parameters on the DC transmission line after the fault are obtained through simulation.Through the analysis of the fault characteristics,the concept of traveling wave method ranging is introduced,and the relevant theories of fault location are introduced,which mainly include the traveling wave process after transmission line failure,the basic theory of traveling waves,the basic concepts of wavelet transform,and wavelet transform.Principle of singular point detection of modulusmaxima.Finally,the current fault location methods for MMC-HVDC DC transmission lines are summarized.A two-terminal fault location method for DC transmission lines that is not affected by wave speed is proposed.Wavelet transform is used to detect and analyze fault traveling waves.The modulus maximum principle detects the sudden change point of the fault traveling wave,and accurately judges the arrival time of the head of initial traveling waveA large number of system simulation verifications for different faults are performed in PSCAD software.The results show that the method can accurately identify the fault traveling wave wave head,and can accurately locate the faults of MMC-HVDC DC transmission lines,verifying the feasibility and correctness of the method. |
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