Electrode line

Many long-distance HVDC systems use sea or ground return for the DC neutral current since this is considerably cheaper than providing a dedicated metallic return conductor on an overhead wire or cable.

HVDC electrodes are used in most bipolar thyristor-based line-commutated converter (LCC) HVDC transmission systems as a means to improve the reliability of the entire system while also reducing the DC voltage and current ripples.

In the event that one of the poles in the bipolar system is faulted, the current path will switch to ground return, thus allowing the system to continue operating at reduced capacity and reducing the possibility that a pole fault will cause a bipolar outage.

The ground current in such schemes can flow in either direction, so the electrodes have to be designed to be reversible, operating either as an anode or cathode.

Concerning voltage-source-converter (VSC) HVDC systems, the main reason to use bipolar configurations, especially when the pole-to-pole DC voltages approach 1 MV, is because when one line is faulted in a monopolar HVDC system, the AC phase-to-ground voltages experience peak voltages twice that of normal operation, calling for double the galvanic isolation that is need for normal operation.