Abstract: It is a major strategic need for our nation to develop the large-scale wind and photovoltaic power in desert or the deep-sea regions. Modular multilevel converter-based high voltage direct current (MMC-HVDC) is considered as the mainstream solution for long-distance power delivery. However, in the grid-forming MMC integrated with renewable generations, on the one hand, the characteristics of faults have significantly changed. On the other hand, the difficulty of relay protection has greatly increased, thereby threatening the secure recovery of the system. We focus on the scenarios of large-scale renewable energy transmission via MMC-HVDC, present the challenges faced by such a system, and describle how to accurately calculate fault characteristics, reliably identify fault zones, and rapidly and safely recove the system. Moreover, we summarize the current research status on three key issues: the inaccurate fault characteristic calculations, unreliable fault area identification, and difficulties in secure fault recovery. Finally, we explore the potential solutions from three aspects: model-based iterative fusion, protection scheme based on or independent of the source characteristics, and the coordination strategy of the control and protection. The research can provide reference for the control and protection of large-scale renewable energy transmission systems in desert region.