Research On Backup Protection Methods Of HVDC Transmission Line

The fault of the HVDC transmission line is one of the common faults of the HVDC transmission project.When HVDC line faults are identified by HVDC line protection,the action signal is sent to the control system to remove the faults and restart the HVDC system.At present,the protection principle installed in existing HVDC transmission projects is subject to the influence of transition resistance and fault distance,and its operation reliability is not high,so the fault on the DC line cannot be removed in time,resulting in unnecessary DC blocking,which affects power transmission and power system stability.Based on evaluating the protection performance of the DC lines revolve round the problems existing in DC line protection in this paper,and this paper deeply excavated the fault information which was included in fault transient from the characteristics of traveling wave propagation,and constructing fast backup protection suitable for high transition resistance faults respectively by utilizing the polarity relationship of the fault components of voltage and current and the characteristics of integrated surge impedance,which effectively improves the reliability and sensitivity of HVDC transmission line protection.The main contents of this paper are as follows:(1)The line protection behavior of actual HVDC project is tested by using the fault recording data of HVDC transmission system,the setting value of DC protection is checked,the operation performance of existing protection is evaluated,and the reasons of protection rejection are analyzed.By summarizing the shortcomings of the existing protection in principle and configuration so that the DC line fast backup protection method with both reliability and sensitivity can be better studied.(2)Based on the analysis of the polarities characteristics of the fault components of voltage and current on both ends of the DC line at different fault positions,this paper proposed a pilot protection algorithm by utilizing the ratio of S transform energy relative entropy of the fault components of voltage and current.When the fault occurs on the DC line,the polarities characteristics of the fault components of voltage and current are same on both sides of the DC line,the difference of S transform energy relative entropy on two sides is smaller and the relative entropy ratio on two sides of the line is smaller;When the fault occurs outside the DC line,the polarities characteristics of the fault components of voltage and current are opposite on both sides of the DC line,the difference of S transform energy relative entropy on two sides is bigger and the relative entropy ratio on two sides of the line is bigger.According to the difference of S transform energy relative entropy ratio of the fault components of voltage and current on both sides of the line when the faults occur,internal faults or external faults can be identified.A large number of simulation results based on PSCAD/EMTDC show that the protection method can quickly and reliably recognize internal faults and external faults of DC lines,and the protection method is not affected by some factors such as transition resistance and fault distance.(3)A new directional pilot protection method based on integrated surge impedance for HVDC transmission lines is proposed in this paper.When the fault occurs on the positive direction at the convertor station,the integrated surge impedance at the measuring point of convertor station is the opposite direction side equivalent impedance.When the fault occurs on the opposite direction at the convertor station,the integrated surge impedance at the measuring point of convertor station is the surge impedance of the DC line.The integrated surge impedance is calculated by initial voltage and current traveling waves at specific frequency on the measuring point of the converter station extracted with S transform.Based on the difference of fault direction,the corresponding criterion are given to distinguish the positive and negative direction faults,and then the internal and external faults can be distinguished by using the fault directions from two terminals.