Impact of Current Limiter Implementation in MMC Control on AC Fault-Ride-Through and System Distance Protection

Modular Multilevel Converters (MMCs) and Voltage Source Converters (VSCs) increasingly dominate the fault characteristics of power systems under AC faults in renewable energy-dominated systems, causing significant challenges in system distance protection. Recent grid codes have specified that grid-connected MMCs are required to have fault-ride-through (FRT) and fault current injection (FCI) capabilities during AC faults. By injecting reactive fault current, the grid voltage can be supported. However, to ensure the safety of MMCs, the grid-side current references have to be limited. There are three types of current limiters: (i) reactive priority, (ii) non-priority, and (iii) active priority, which will cause different phase shifts on the converter injected current and thus affect FCI, FRT of MMCs and distance protection performance. Existing literature either only considers reactive current priority implementations of the current limiter or does not mention the priority selection during the current limitation. Through analysis and case studies, this paper examines the impact of those three current limiters on internal energy balancing during FRT and distance protection under various fault scenarios and operational conditions. The findings reveal a complex interaction between current limiter implementations, MMC fault behaviour, and system protection performances, offering insights for improving the coordination between the control and protection systems of MMC-HVDC.