A non-unit boundary protection of DC line for MMC-MTDC grids

Due to the low-impedance characteristic, the multi-terminal high voltage direct current (MTDC) grid based on modular multilevel converter (MMC) requires protection to detect and discriminate line faults within several milliseconds, which is a serious challenge for protection. Therefore, this paper proposes a non-unit line protection scheme based on the initial line-mode voltage traveling waves (ILVTWs). Based on the analysis of the traveling wave (TW) propagation characteristics, the transfer functions of ILVTWs under typical internal and external faults are derived. By comparing the differences of the transfer functions in frequency domain, the corresponding protection principles are developed. Besides, a method to determine the protection threshold is elaborated, which is independent of simulation. Extensive simulations show that the proposed protection scheme can accurately operate with high sensitivity and reliability.