Monopolar and bipolar HVDC transmissions.
Most HVDC transmissions are point to point transmissions using overhead lines or submarine cables or a combination of lines and cables.
Many of the cable transmissions are monopolar with only one metallic conductor between the converter stations and using the ground as the return path for the current.
Most overhead line transmissions are bipolar, i.e. they use two conductors of opposite polarity (one positive and one negative). A bipolar transmission is in fact a double circuit transmission, since one pole can continue to transmit power when the other pole is out of service.
