Research On Traveling Wave Fault Location Method Of MMC-HVDC Transmission Line

In recent years,flexible HVDC transmission technology has been gradually promoted.However,flexible HVDC transmission lines are prone to short-circuit faults due to long distances and complex terrain,which in turn affects the stable transmission of electric energy.Therefore,reliable fault ranging is of great significance and practical value to maintain the stable operation of the system.Most of the flexible HVDC transmission projects are based on high-frequency traveling waves to achieve fault location,and their ranging accuracy is highly dependent on the accurate calculation of traveling wave velocity which has typical frequency-dependent characteristics.Up to now,although scholars have made many achievements in traveling wave fault location,there are still the following problems:Firstly,the frequency dependent traveling wave refraction and reflection will occur at the boundary of the converter stations on both sides of transmission line.the traditional traveling wave location algorithm often ignores the influence of the frequency dependent characteristics of the boundary on the spectrum obtained from the time-frequency analysis of the traveling wave signal,and the spectrum distortion will affect the fault location results;Secondly,for the overhead transmission lines with long distance,the traditional traveling wave ranging often ignores frequency-changing characteristics of transmission line,and the dispersion effect of the traveling wave will affect traveling wave velocity,thus affecting the ranging results;Thirdly,for the flexible DC transmission line with hybrid multi-terminal topology,the influence of the frequency dependent characteristics of the boundary on the spectrum is more complex.How to correct the distorted spectrum and apply it to the fault location principle needs to be studied.Accordingly,the main research contents of this paper are as follows:First,for the traditional two-terminal ranging principle,the calculation of wave velocity is closely related to the acquisition of the instantaneous frequency of the traveling wave.The frequency spectrum distortion of the traveling wave caused by the frequency-dependent characteristics of the boundary reduces the accuracy of the instantaneous frequency acquisition,which has a great impact on the ranging results.Therefore,based on the analysis of boundary frequency-dependent characteristics of two-terminal flexible DC transmission,this paper proposes a two-terminal fault location method based on boundary frequency correction algorithm.The time-frequency variation diagram of the traveling wave is obtained by S-transform for the obtained traveling wave signal,and the exact instantaneous frequency of the traveling wave is obtained by correcting the traveling wave spectrum according to the analysis,from which the corresponding traveling wave velocity can be obtained,and the fault distance is calculated based on the two-terminal traveling wave ranging principle.The effectiveness of the proposed method is verified by simulation of different fault influencing factors.Second,the frequency changing characteristics of the transmission line will cause the dispersion effect of traveling wave on the actual line.Based on the frequency variation parameters of the transmission line,this paper analyzes the dispersion variation law of traveling wave.For the spectrum distortion caused by dispersion,this paper proposes a two-terminal ranging method based on dispersion frequency correction algorithm.This method uses the traveling wave spectrum of S transform to calculate the dispersion correction factor,and obtains the frequency after dispersion correction,and calculates the fault distance based on the two-terminal ranging principle.The simulation results of different fault influencing factors show that the proposed method can effectively correct the traveling wave dispersion frequency and maintain high fault location accuracy compared with traditional fault location methods.Third,the branch connection points and boundary converter stations of the hybrid multi-terminal DC transmission line will affect the spectrum of the traveling wave,and the spectrum distortion will affect the calculation of the traveling wave velocity,which will cause great error to the fault location.In this paper,a multi-terminal fault location method based on frequency correction is proposed to reduce the location error.In this paper,the superposition theorem is used to analyze the fault state of the multi-terminal hybrid topology.Based on the fault transient analysis,a fault location method suitable for the hybrid multi-terminal topology is proposed.The method proposed in this paper firstly uses S transform to obtain the corresponding traveling wave velocity at each end,and then realizes the accurate location according to the fault location principle.The proposed multi-terminal fault location method is verified by the simulation of different fault influencing factors,and maintains higher fault location accuracy compared with traditional fault location methods.