Research On The Adaptability And Countermeasures Of Relay Protection Under Different Line-Touching Faults In AC/DC Hybrid System

HVDC transmission technology is developing rapidly,and the world’s largest AC/DC hybrid power grid has been built in China.However,due to the limitation of transmission corridors,the short-distance installation of transmission lines of different systems is inevitable,which leads to a greater risk of line-touching faults at close electrical distances between lines.After a line-touching fault occurs between different system lines,the electrical quantities of each faulty system interact through the fault point,showing characteristics that are different from single-system line faults,which poses severe challenges to the performance of relay protection.In this paper,the AC-AC line-touching faults that occur between different AC systems erected on the same tower,the AC-DC line-touching faults that easily occur at the intersection of AC and DC lines are the research objects,and the characteristics of the line-touching faults、andadaptability and countermeasures of related relay protections are discussed.Firstly,the fault characteristics of AC-AC line-touching fault and AC-DC line-touching fault are analyzed in detail.For AC-AC line-touching faults,considering the effects of strong magnetic coupling between lines erected on the same tower,the sequence current and phase current characteristics and their influencing factors under various types of faults are analyzed;for AC-DC line-touching faults,the fault characteristics of the DC side and the AC side of the fault point are analyzed separately.Aiming at the nonlinear problem of the converter,based on the basic principle of converter conduction,the flow characteristics of power frequency electrical quantities at the converter under different protection action sequences are analyzed.Secondly,it analyzes in detail the adaptability of current phase selection components under various AC-AC line-touching faults,and proposes a new fault phase selection scheme.Based on the phasor method,the adaptability of the current phase selection element is analyzed.Studies have shown that when multi-phase transmission line in a single system have a line-touching fault at the same time,the current phase selection element has insufficient adaptability;in order to solve this problem,using the basic change law of the fault phase current waveforms,a fault phase selection scheme with phase current waveform autocorrelation coefficient and improved autocorrelation coefficients based on the correlation of Pearson signals is proposed,and equipped with sequence component auxiliary phase selection criterion to realize the correct phase selection under the influence of strong magnetic coupling.Thirdly,the adaptability of the AC-DC line-touching protection of the DC line under the AC-DC line-touching fault is analyzed,and put forward the protection improvement criterion.Aiming at the problem that the nonlinearity of the converter makes it difficult to calculate the fault electrical quantity,a piecewise linearization analysis method of the converter fundamental frequency equivalent impedance is proposed,By comparing and analyzing the analytical expression of the fundamental frequency electrical quantity of the protection before and after the phase shifting of the main protection of the line,the influence of the phase shifting of the main line protection on the adaptability of the AC-DC line-touching protection is explored.Aiming at the problem that the AC-DC line-touching protection cannot quickly block the fault pole after the phase shift action,considering the characteristics of power frequency voltage and pole wave changes comprehensively,a new criterion for protection improvement is proposed.Finally,the risks of existing protection actions when different types of AC-DC line-touching faults are explored,and a new protection action sheme is proposed.After comprehensively considering the coordination relationship between the relevant protections of the AC and DC systems after the AC-DC line-touching fault,it is pointed out that the DC line cannot achieve adaptive restart under the AC-DC line-touching fault,and the AC system has the risk of running with the fault;for this reason,it is recommended to add AC-DC line-touching protection criterion on the AC side,and then propose a new AC-DC line-touching fault protection scheme based on the coordination and cooperation of the AC and DC system protection.