The protection system is an integral part of an electrical transmission system. Previously, various protection methods have been proposed for AC systems and work has been done for years to achieve better performance. In recent decades, HVDC transmission has been introduced as a solution for long-distance transmission and offshore transmission, thanks to developments in power electronics technology. Fast and flexible control, large transmission capacity, economic justification for distances over 500 km (depends on power electronics technology) and less busy right-of-way (RoW) than HVAC transmission for a certain transmission capacity power[1], are some advantages of using HVDC transmission. A statistical analysis on an HVDC transmission system in China shows that 36.8% of the outages of 114 valve sets were caused by faults in the line protection zone[2]. Therefore, a reliable protection method can prevent incorrect fault detection and consequently reduce the entire power failure. Differential protection is one of the traditional solutions used for AC transmission. in [3] a typical SIEMENS HVDC line protection configuration was studied which introduced differential protection as backup protection. Improved differential protection of Current Source Converter (CSC)-HVDC transmission lines was proposed in [4], where a composite of a blocking unit and a newly defined differential current criterion was used for fault detection . [5] He defined a signal distance between the currents on the rectifier side and the inverter side where it is possible to distinguish the external fault from the internal one. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay The effect of capacitive current and the problem of communication channel requirements have been mitigated in [6], [7]. Transient power and other combinations of double-sided voltage and current measurements have been studied in [8]–[10]. The presence of a smoothing reactor and DC-side filters at both ends of a CSC-HVDC link allows implementing some non-unitary selective protection methods[11] such as using the impedance characteristic of the smoothing reactors and filters DC side under fault conditions which was carried out in [2], [12], [13]. Traveling wave protection (TWP), used as primary protection [14], can benefit from some methods such as principal component analysis (PCA), wavelet transform (WT) and so on [15]-[17] , but the attenuation and distortion of the traveling wave caused by the fault resistance and fault location should be taken into account. Although some protections have been suggested for VSC-HVDC systems, these can also be applied to CSC-HVDC transmission lines. (Methods based on current and voltage derivatives can also be mentioned, which possibly the references in the introductory part reach beyond 23). It should be noted that almost all cited research used the concept of threshold as a criterion which is usually obtained non-analytically (worst case based). There is usually a trade-off between safety and reliability in protection systems that causes inaccuracies in threshold-based methods. as of March 21, 2005, the primary protection of the Tian-Gaung HVDC system failed to detect a high-impedance fault because the amplitude did not reach the threshold value [18], and the backup protection refused to act due to of current fluctuations. To overcome the.