Analysis of fault characteristics in transmission lines of grid-forming MMC-HVDC system

With the advancement of the "carbon peak and carbon neutrality" strategy, modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission technology plays a crucial role in scenarios such as renewable energy transmission and weak-grid support. Currently, the grid-following control adopted by MMC struggles to adapt to weak-grid conditions, while grid-forming control enables converters to provide active support capabilities through self-synchronisation mechanisms, serving as a key technology for ensuring the secure and stable operation of new power systems. This paper focuses on an MMC-HVDC transmission system with grid-forming control (GFM), modelling its control architecture and analysing its fault ride-through strategies. The phasor plane diagram method is employed to examine the characteristics of electrical quantities following faults in transmission lines, comparing the similarities and differences with grid-following control (GFL). Taking distance protection as an example, the impact of fault ride-through strategies for grid-forming converters on protection reliability is analysed. Finally, simulation-based analysis verifies the conclusions, providing theoretical support for optimising AC line protection design in new power systems.