Non-unit protection scheme for MMC-HVDC transmission lines based on waveform characteristics of instantaneous amplitude-frequency curve

• This paper introduces an innovative definition of the instantaneous amplitude-frequency curve (IAFC) and derives its analytical expressions for various fault types. • Theoretical analysis based on the IAFC expressions indicates distinct distribution characteristic differences across different fault types. • A novel protection based on the distribution characteristics of the IAFC is proposed. The protection performance is thoroughly verified through extensive simulations. The principle is inherently unaffected by transition resistance, and a clear method for setting the protection criteria is provided. To address the limitations of existing protection schemes for Modular multilevel converter based high voltage direct current (MMC-HVDC) transmission lines, such as poor tolerance to high-resistance faults and the lack of effective setting principles for protection criteria, this paper proposes a non-unit protection method based on the distribution characteristics of the instantaneous amplitude-frequency curve (IAFC). First, the IAFC expressions are derived from the complex frequency-domain models under internal ground fault (IGF), external ground fault (EGF), lightning interference (LI), and lightning fault (LF) conditions. By analyzing the extremum features of the IAFCs under different faults and disturbances, a lightning identification method is proposed to effectively distinguish between LI and LFs. Furthermore, based on the differing high-frequency attenuation characteristics of IAFCs under IGF and EGF conditions, the linear fitting is applied to the high-frequency segment of IAFC, and a goodness-of-fit-based criterion is introduced to differentiate IGFs and EGFs. A complete protection scheme is then presented. The proposed method is theoretically immune to fault resistance, and the analytically derived IAFCs provide a clear basis for protection setting. Extensive simulation results in PSCAD/EMTDC validate that the proposed scheme effectively distinguish IGFs, EGFs, LI, and LGs, while demonstrating strong noise immunity, fast response, and high tolerance to fault resistance.