A New Protection Scheme for HVDC Transmission Lines Based on the Specific Frequency Current of DC Filter

High-voltage direct-current (HVDC) transmission line protection is essential to the safety and reliability of modern power system with the wide application of HVDC transmission technologies. Based on the characteristics of specific frequency current (SFC) of dc filter, a novel pilot protection scheme for HVDC transmission lines is proposed in this paper. According to the harmonic equivalent circuit and control strategy of HVDC system, the SFC characteristics of dc filter are theoretically analyzed for internal and external faults in the fault steady period. In addition, these characteristics are used to construct a backup protection scheme to identify different fault types. A +/- 800 kV UHVDC system built in PSCAD/EMTDC is used to verify the accuracy and validity of the proposed protection. Comprehensive simulations show that the proposed protection scheme can not only accurately identify internal and external faults, but is also insensitive to high fault resistance. Furthermore, the proposed protection does not require a high-frequency sampling rate and data synchronization of both ends, making it valuable and prospective for applications inHVDC transmission system.