Research On Relay Protection Scheme For HVDC Transmission Lines

HVDC transmission technology plays an important role in China's West-East Power Transmission and power system networking projects,because it has obvious advantages in long-distance large-capacity transmission and power system networking.The length of HVDC transmission line is generally long,which causes a high failure probability,so highperformance protections for HVDC lines are an important guarantee for the safe operation of DC systems.However,there are still many shortcomings in traditional protection scheme HVDC transmission lines: The main protection can't respond to internal faults with large transient resistances,and the delay of the backup protection is too long,which causes that there is no protection principle which can respond to faults for a long time after faults occur.To solve this problem,four kinds of protection schemes for HVDC lines are proposed in this paper.Based on the cooperation of these protections,a overall protection scheme of HVDC transmission lines is proposed.The main contributions are follows:To solve the problem that the delay of traditional backup protection is too long,a novel differential protection principle based on instantaneous compensation of distributed capacitance current is proposed.Theoretical analysis shows that the distributed voltage along the line can be approximated as linear distribution,after electrical quantities at both ends of tht DC lines are processed by suitable low-pass filters.Based on the linear distributed voltage,the distributed capacitive current can be calculated.After removing this capacitive current,internal faults can be distinguished by external faults.If the cutoff frequency of the filter is further depressed,the fault line can be selected by comparing the ratio of the differential currents of the positive and negative lines.Simulation analysis shows that the novel protection can identify internal faults and select fault lines quickly and reliably.Besides,it has a strong ability to respond to transient resistances.the performance of the novel protection is significantly better than the traditioanl backup protection for DC lines.Aiming at the problem that the sensitivity of distributed capacitor current compensation differential protection is easily affected by the accuracy of capacity parameters,a novel differential protection principle for HVDC lines based on current low-frequency components is proposed.As the frequency of the voltage and current signals decreases,the value of the distributed capacitance current along the line decreases.Therefore,when the cut-off frequency of the low pass filter is low enough,the distributed capacitor current can be ignored.Based on that,the filtered differential currents of the two-pole lines can be used to identify internal faults,avoiding the influence of line parameter errors.The simulation analysis shows that the novel protection can also respond to internal faults reliably,and it has a strong ability to respond to transient resistances.At present,the pilot directional protection for HVDC lines proposed by the relevant literatures mostly has the disadvantages of poor tolerance to transient resistances and the requirement of high sampling rate.To solve the problem,a novel pilot direction protection principle for HVDC lines based on transient current comparison is proposed.In practical engineering,measurement units are installed on both sides of the high-voltage DC bus,so measurement information on both sides of the DC bus can be used to construct DC line protections.The fault direction can be identified by comparing the fault transient currents on both sides of the DC bus because of the high-frequency filtering characteristics of the DC filters.A directional component based on transient current comparison is proposed,and internal faults can be distinguished from external faults based on discrimination results of the direction components at both ends of DC lines.In addition,the fault lines can be further determined by comparing the transient currents of both lines.The novel directional protection does not require data synchronization at both ends of DC lines.The simulation results show that the novel protection can identify faults quickly and reliably,and it also can respond to high impedance Ground Faults.Traveling wave protection and differential undervoltage protection for HVDC lines have the disadvantages of lack of theoretical basis and setting value depending on simulation.Aiming at this problem,a novel two-step distance protection principle for HVDC lines is proposed.Theoretical analysis shows that the DC transmission lines can be approximated as ?-type circuit model after electrical quantities at both ends of tht DC lines are processed by suitable low-pass filters.Based on the ?-type equivalent circuits of the DC lines,the fault distances can be obtained by iterative calculations,and used to distinguished internal faults from external faults.The simulation analysis shows that the novel distance protection can respond to single-pole-to-ground faults with low-resistances and pole-to-pole faults reliably.To solve the problems of protection scheme for HVDC transmission lines,based on the above four protection principle for HVDC lines proposed in this paper,a new overall protection scheme for HVDC transmission line is proposed.In the new overall protection scheme,the differential protection based on distributed capacitance current compensation and the differential protection based on current low-frequency components can respond to faults for a long time,but their reliability depends on data synchronization at both ends of the lines.The pilot direction protection based on transient current comparison does not rely on data synchronization,but it cannot respond to faults for a long time.Compared with the above three protections,the reliability of the two-step distance protection is not affected by communication.In order to verify the performance of each DC line protection principle,the relevant dynamic model verification experiment was carried out.According to the parameters of transmission tower and DC line in Wudongde-Guangxi HVDC transmission project,the dynamic model of the DC lines was made.Simulation and dynamic model experiments verify the correctness of the designed dynamic model.Finally,the performance of novel protections proposed in this paper is verified by dynamic model experiments.