| With the rapid development of power electronics technology,the MMC converter system with fully controlled switching devices has been applied in practical engineering.There are differences in principle and structure between MMC-HVDC system and traditional DC system,at present,the research on MMC converter is mainly focused on the design of the system,control strategy and so on.The fault feature analysis and the fault location of the transmission line need to be further studied.For this reason,this paper focuses on the fault analysis of the MMC-HVDC system and the fault location of the transmission line,aiming at finding out the fault characteristics of the MMC-HVDC system and putting forward the corresponding method of transmission line location.Combined with the MMC-HVDC system model built by PSCAD/EMTDC,the fault types and fault points of the system are introduced.With the simulation analysis,the fault characteristics of the AC side,converter and DC lines and the response characteristics of the system are analyzed;And simply analyzes and compares the voltage of the upper and lower legs of the converter under different fault types.Calculating the frequency variation parameters of transmission lines are by its physical parameters.Analyzing the frequency variation characteristics of the parameters such as wave impedance,attenuation coefficient and wave speed,and the linear relationship between the transmission line wave velocity and the fault distance is obtained by using the simulation model in the condition of different fault distance.The traveling wave characteristics and influencing factors of transmission lines are analyzed,and the applicability of two classic traveling wave front calibration methods of wavelet transform modulus maxima and HHT is compared.A fault location method for MMC-HVDC overhead lines is proposed: analyzing the three traveling wave front information of the double end,proposing a fault section discrimination method based on the information of double end wave front,and combining the time difference of different scales of wavelet decomposition,a multi scale time difference traveling wave fault location method which is not affected by the wave speed is proposed.The simulation results show that the location accuracy of the proposed method is higher than that of the traditional two terminal fault locations,and the method is suitable for cable line fault location.For the cable and overhead transmission lines,analyses the propagation characteristics of voltage and current traveling waves of two transmission lines with diferrent MMC-HVDC topology.In the analysis of the fault location in the overhead line section,it is impossible to identify the second traveling wave front properties,but it done the fault at the cable.On the basis of identifying the nature of the second traveling wave front,the fault line section discrimination method based on the multi-scale morphological spectrum is proposed,and the fault point is prejudged in the overhead line area by the line mode and the zero-mode wave speed difference,with the second traveling wave front are identified,achieving the single ended fault location,second traveling wave front are identified.It is verified by simulation that the proposed method is accurate. |
