OK, most of the following is textbook stuff on #HVDC but just a quick reference for what a "low-voltage metallic return" - the faulty part of #Basslink - is used.
Basslink is a monopolar HVDC system and the original design was based on an earth return. This is quite typical of monopolar #HVDC systems as it reduces cost (requires only one cable) and losses (again because of one cable)
ptd.siemens.de/IEE_HVDC_0306.…
The reasons #Basslink was eventually designed with a metallic return were mostly environmental (see link above) to avoid
"- Corrosion of long metallic structures
- Electrolysis effects of sea electrodes
- Electric and magnetic field effects"
Some other reasons to choose a metallic return might by concerns over interference, particularly to nearby communications line (These is a metric called Telephone Interference Factor - TIF) exactly for that, and railway interferences.
Drawbacks of the metallic return option in the system:
- Cost
- Losses
- Voltage drop - this translates to different transformer requirements, and
- Outages
Without having additional information - hopefully these will be made available later - besides repairs in the cable itself would be anything related to overvoltage protection - e.g. surge arresters, fuses, any spark gaps that are present that #converter station
This would be because faults in the metallic return can create overvoltages. Usually there are protections at the ends exactly for that reason
Figure is only for reference - from the #UHVDC transmission systems textbook
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