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

• Published in: International journal of electrical power & energy systems 2020-03, Vol.116, p.105538, Article 105538
• Main Authors: Zhang, Shuo, Zou, Guibin, Wang, Chuanjie, Li, Jun, Xu, Bin
• Format: Article
• Language: English
• Subjects: Initial line-mode voltage traveling wave (ILVTW); Modular multilevel converter (MMC); Multi-terminal high voltage direct current (MTDC) grid; Non-unit boundary line protection; Wavelet transform
• ISSN: 0142-0615; 1879-3517
• DOI: 10.1016/j.ijepes.2019.105538

•The proposed non-unit line protection has high sensitivity and reliability.•The sampling frequency can be chosen flexibly according to Section 4.•The action time of the protection is within 1.3 ms.•The method to obtain the protection threshold is elaborated in Section 5. 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.