Study On Fault Location And Classification Of DC Line In MMC-HVDC System

As the power technology developed rapidly in recent decades,modular multilevel converter(MMC)is widely used.MMC based high voltage direct current(MMC-HVDC)has become a research hotspot in the field of transmission technology.The technology has the advantages of large capacity,low cost as well as convenient control.Therefore,it has been widely used in the situation of largescale power grid interconnection,isolated island power supply and new energy power generation.If the MMC-HVDC system can’t work normally,it will have a huge impact on the stable operation of the power system.Domestic and foreign scholars mainly focus on the research of MMC-HVDC system fault in the converter and AC system,and the research of DC transmission line fault focuses on the control and protection after line fault.Therefore,it is necessary to study the fault location and classification of HVDC transmission lines.Firstly,this thesis analyzed the basic structure,working principle,sub module capacitor voltage sharing strategy and converter station modulation mode of MMCHVDC system,analyzes and calculates its main parameters.The simulation model of double-terminal 21 level MMC-HVDC system is built on PSCAD /EMTDC simulation platform.Secondly,according to the main fault types of MMC-HVDC transmission lines,the fault characteristics under different fault conditions are analyzed,and a fault location method for HVDC transmission lines in MMC-HVDC system based on improved variational mode decomposition(VMD)and S transform is proposed.The penalty factor ? and mode number K of traditional VMD need to be assigned manually,which will lead to inaccurate decomposition results.In order to obtain better decomposition results,the optimal values of ? and K are obtained by alternately optimizing parameters.In order to get the accurate time when the fault traveling wave reaches both sides,S-transform is used to analyze the decomposition results.In order to eliminate the influence of wave speed on fault location accuracy,a fault location method of double-terminal transmission line is adopted,which is independent of wave speed.The simulation results show that the fault location method is not affected by the fault location,has strong resistance to the transition resistance,and has high positioning accuracy.Finally,in view of the need to identify the fault types in practical engineering applications,naive Bayes classification algorithm is introduced into the research of HVDC transmission line fault classification in MMC-HVDC system,and the discretization process of the algorithm is improved.In MATLAB simulation platform,the effect of naive Bayes classification algorithm is simulated and verified.The simulation results show that the naive Bayes classification algorithm can effectively identify the fault types of HVDC transmission lines in mmc-hvdc system,and has high accuracy and good adaptability.