The Research About New Pilot Protection Of UHVDC Transmission Line

In the field of long distance,large capacity power transmission,UHVDC transmission has obvious advantages than AC transmission.The development of UHVDC transmission is of great importance because of the wide area of China and the inverse distribution of energy and load.UHVDC transmission has the characteristics with large capacity,long distance and high voltage.So the reliable relay protection is the key to ensure the safety of UHVDC transmission.However,the operation data show that the accuracy rate of relay protection for UHVDC transmission project is low,especially to the DC line which has the highest failure rate.Therefore,it is high time to put forward new protection principles of UHVDC transmission line.In this paper,by analyzing the transmission characteristics of current travelling waves,the U-I characteristics of the system fault and the control characteristics of the converter valves,new protection principles of UHVDC transmission lines are proposed as follows:(1)A new protection principle is proposed after analyzing the changes of current and the polarities of it at the two ends of the DC line.This protection principle uses current difference as starting criterion,and current polarities as protection criterion.When the fault occurs near the non neutral point of the DC line,the traveling wave propagation delay will cause the data of the two ends of the line not synchronized,and then cause the protection to refuse to action with internal faults.By analyzing the mechanism of traveling wave transmission,a scheme of compensating the delay error is proposed.(2)A new principle of pilot protection based on U-I characteristic is put forward by analyzing the moving locus of points in U-I plane which use voltage and current as coordinates.It is found that the U-I dynamic trajectories go through different regions under the condition of internal faults and outernal faults.Aiming at the difference,the protection criterion is built on rectifier side and inverter side respectively.When the fault occurs,both ends use their own voltage and current information to get their respective fault directions,and then find out the location of the failure.(3)Based on the analysis of the control system of UHVDC,a new principle of pilot protection based on internal trigger angle of converter is proposed.Because of the regulating function of the control system,the triggering control angles of the rectifier side and the inverter side will only increase at the same time with internal faults.Compared with the traditional principles which use voltage and current to construct protection criterions,the principle only uses the internal control parameters.So it has the characteristics of high reliability,fast action speed,good sensitivity and strong resistance to transition resistance.The model of ±800k V UHVDC transmission is built based on PSCAD software and the new protection principle is simulated through the model.The simulation results show that the protection principles can discriminate internal faults and outernal faults well.