With the continued advance of China's carbon peaking and carbon neutrality goals and the rapid development of high-voltage direct-current transmission based on the modular multilevel converter (MMC-HVDC)

receiving-end power grids are experiencing unprecedented changes in dynamic characteristics. A rising share of power-electronic equipment makes them vulnerable to rapid frequency decline

voltage fluctuations

and multi-band oscillations after disturbances. As a key technology for long-distance power transmission and regional renewable-energy integration

MMC-HVDC is crucial for active frequency and voltage support and oscillation control. Addressing these operational challenges

this paper analyzes the evolution mechanisms and instability risks of receiving-end grids from the perspectives of frequency

voltage

and resonance stability

and reviews progress in MMC-HVDC active support for inertia and frequency regulation

voltage

and damping. Then

this paper summarizes the coupling and common constraint boundaries among these three support requirements in terms of characteristic time scales

converter current-margin allocation

and control/communication delays. Finally

this paper discusses challenges and key issues at the grid

MMC-HVDC system

and equipment levels

providing theoretical and technical guidance for a coordinated multi-level support mechanism encompassing the grid

MMC-HVDC

and equipment.