Polarity of current area based active injection protection scheme for hybrid MMC-HVDC grids

With the massive increase of power electronic devices to facilitate renewable integration, the system’s fault characteristics will be dramatically changed due to the different control strategies adopted by converters. As a result, the dependability and security of the traditional protection can be severely compromised. Considering the high controllability of power electronic converters, this paper presents an active injection protection scheme to effectively identify the fault. By analyzing the traveling wave (TW) refraction and reflection characteristics under different fault conditions, the variation of the measured terminal current caused by the TW is revealed. Then, a rectangular wave is injected by the hybrid modular multilevel converters (MMC) for a short time after the fault ride-through process. By introducing the concept of the polarity of the current area, the proposed protection scheme only needs to measure the terminal current without any complex signal extracting and processing algorithm to identify the internal and external faults. The feasibility and effectiveness of the proposed protection scheme are verified by realistic case studies with simulation conducted in the PSCAD platform.