There are no technical limits for the length of a HVDC cable.
In a long AC cable transmission, the reactive power flow due to the large cable capacitance will limit the maximum possible transmission distance. With HVDC there is no such limitation, why, for long cable links, HVDC is the only viable technical alternative.
The 580 kilometer-long NorNed link is the longest underwater high-voltage cable in the world.
Link to: Power Cable Systems