Research On Differential Protection Of AC Transmission Line In AC/DC Hybrid System

With the wide application of both AC and DC power transmission modes,the interaction between AC and DC systems becomes more and more closely,and the interaction between them becomes more and more complicated.In the AC-DC hybrid system,failure of the AC system side will cause the commutation failure of the invert station to occur,and the DC current fed to the AC side has a complex coupling effect.When the commutation bus fails and commutation fails,AC line differential protection malfunction occurs.Therefore,it is necessary to study the differential protection of AC transmission lines that is not affected by the commutation failure.Firstly,using the PSCAD simulation software,a AC-DC hybrid system and a pure AC transmission system model for a 500 kV AC grid fed by a double-circuit AC-DC transmission line with a ±800kV bipolar dual 12-pulse UHVDC transmission system is built.The electrical characteristics of the AC-DC hybrid system and the pure AC transmission system in the actual project are simulated and the electrical characteristics of the two faults are compared and analyzed.Secondly,from the perspective of the working principle of the 6-pulse converter,analyze the mechanism of commutation failure and classify the failure of commutation.Exploring the severity of commutation failure in both the faults of the commutation bus and the failure of the AC transmission line study.When the commutation bus fails,as the commutation failure degree gradually increases the current side current fed to the AC side current distortion is deepened,and the amount of differential protection of the AC transmission line is gradually greater than the braking amount,resulting in the traditional AC line differential protection malfunction.Finally,a new differential protection method that is not affected by commutation failure is proposed by using commutation bus voltage monitoring and current phase fault characteristics on both sides of the AC line.Detection of commutation failure based on the relationship between the commutation bus voltage and the severity of the commutation failure.Utilize the phase difference of the currents on both sides of the AC transmission line to accurately determine whether there is a fault in the AC transmission line zone.A large number of simulations have shown that this method can operate correctly under various fault conditions and can effectively and quickly protect the AC line.