Flexible Dc Transmission Line Protection Scheme Based On Energy Spectrum Matrix And Traveling Wave Similarity

Flexible DC transmission line has long distance,needs to cross different areas,poor working environment and prone to failure.For the fault of flexible DC overhead line,due to the lack of effective current limiting measures,the capacitor of converter station discharges rapidly in a short time,which makes the fault current rise rapidly.It is required that the flexible DC main protection complete the fault identification within 2～5ms.At present,the flexible DC projects put into operation directly adopt conventional DC protection,which is difficult to meet the harsh speed requirements of flexible DC protection,Therefore,it is of great significance to study the main protection scheme of flexible DC transmission line which can act quickly and accurately.At the same time,high resistance grounding,noise interference,lightning interference and other factors also affect the correct action of the protection,resulting in low protection reliability.At present,there is little research on the backup protection scheme of flexible DC transmission line.It is urgent to study a safe and reliable backup protection scheme of flexible DC transmission line to provide protection in case of failure of the main protection.According to the above protection requirements and interference factors,starting from the fault characteristics of flexible DC transmission line,this paper studies two protection schemes,which are used as the main protection and backup protection of flexible DC transmission line.The research contents and results of this paper are as follows:Firstly,this paper theoretically analyzes the propagation process of traveling wave in flexible DC transmission line,obtains the attenuation coefficient,wave impedance,wave velocity and other parameters of traveling wave according to the circuit principle,then analyzes the refraction and reflection characteristics of traveling wave,and calculates the refraction and reflection coefficients.Finally,the fault characteristics of flexible DC lines under different fault types are analyzed,and the analytical formula of initial voltage traveling wave is deduced.It provides a theoretical basis for the time-frequency analysis of fault traveling wave and the design of protection scheme.Aiming at the problems of existing flexible DC line protection in high transition resistance,noise interference and failure of remote high resistance fault in the area,a single end protection scheme of flexible DC transmission line based on S-transform traveling wave energy spectrum matrix similarity is proposed.By analyzing the boundary effect of current limiting inductance in flexible DC power grid,the difference of fault voltage traveling wave in time domain and frequency domain between inside and outside the area is obtained.The energy spectrum matrix is constructed by S-transform to increase the difference of fault energy spectrum distribution inside and outside the area.Through principal component analysis,the dimension of energy spectrum matrix is reduced,the dimension of sample matrix is reduced,and the storage space is saved.Using the normalized cross-correlation coefficient,the similarity between the fault energy spectrum matrix and the fault sample matrix is obtained,and the single ended protection criterion of flexible DC transmission line is constructed to distinguish the faults inside and outside the area.Considering the factors of poor working environment and vulnerable to lightning stroke of overhead transmission line,the identification criterion of lightning interference is constructed.The simulation model of Zhangbei flexible DC system built by PSCAD/EMTDC is used.The simulation results verify the effectiveness of the protection scheme,which is not affected by fault location,high resistance,noise interference and so on.Aiming at the failure of traveling wave main protection scheme under low signal-to-noise ratio and high transition resistance,a backup protection scheme for DC transmission line based on wavelet threshold denoising and improved editing distance algorithm is proposed in this paper.According to the difference between noise signal and fault traveling wave signal in frequency domain,wavelet denoising is used to preprocess the collected fault traveling wave signal.According to the protection principle of waveform similarity of two terminal traveling waves,the improved editing distance algorithm is used to overcome the influence of data synchronization and abnormal value,realize the accurate calculation of similarity between fault traveling waves on both sides of the line,and establish the fault identification criterion of flexible DC transmission line.Simulation experiments on Zhangbei flexible DC simulation system verify that this scheme can accurately identify faults inside and outside the area.Compared with the original editing distance algorithm and other two terminal protection schemes,this scheme has better effects in anti-noise interference,withstanding transition resistance,withstanding data synchronization and so on.