Control and Recovery of DC Faults in Overhead Line UHVDC Systems: A Comparison of Full-Bridge and Half-Bridge MMCs

(IEA), global electricity demand is projected to rise by approximately 40–50% by 2035. This anticipated growth, combined with large-scale renewable energy integration, will necessitate a substantial expansion of transmission capacity worldwide in the coming decades. Among the various solutions for enhancing transmission capacity, Ultra-High-Voltage Direct Current

(UHVDC) transmission, defined as HVDC systems operating at ±800 kV or above, has emerged as a highly promising option. In existing HVDC projects in Europe, cable-based solutions are often adopted in routing scenarios, such as urban areas requiring high visual amenity, river crossings, or constrained substation entry corridors, where undergrounding becomes necessary. These practical considerations influence the selection of cable and overhead line (OHL) options like HVAC systems. However, for high-voltage and bulk-power

UHVDC transmission, cable-based solutions remain generally impractical due to the extreme challenges associated with insulation design, manufacturing complexity, and prohibitive costs.

Consequently, OHLs remain the predominant choice for UHVDC applications at the present stage. Since OHLs are more susceptible to transient faults and the UHVDC is primarily used for bulk power transmission, it is necessary to address not only fault interruption but also rapid system recovery to quickly restore the power transfer.

In this paper, bipole UHVDC systems using Half-Bridge (HB) and Full-Bridge (FB) Modular

Multilevel Converter (MMC) configurations are modelled in Real-Time Digital Simulator. A detailed recovery framework, comprising steps of fault clearance, de-energization, permanent fault identification, and DC voltage and active power recovery, has been proposed to evaluate the recovery of the FB- and HB-MMC-based UHVDC systems. Simulation results demonstrate that, compared with the HB-MMC, the FB-MMC-based UHVDC systems can achieve a faster recovery, thereby enhancing overall system resilience. The findings provide critical insights to support technology selection for future large-scale UHVDC projects.