Feasibility of ultra-long cable-based HVDC links

HVDC transmission technology has opened up the ability to transmit large amounts of power over very long distances, connecting renewable generation resources with distant load centres, and opening up the possibility of intercontinental transmission. It is often stated that HVDC does not have a practically relevant transmission distance limit. This is true for overhead line based ultra-high voltage (UHV) systems because both the effective conductor cross-section as well as the transmission voltage can be increased relatively easily. It may not be true for cable based HVDC systems in which both the transmission voltage as well as the maximum conductor size are more limited. Several ultra-long cable based HVDC projects of several 1000s of km are in development today. This paper explores several aspects of such systems that could impose limits on the maximum feasible transmission distance with todays technology. Aspects covered are HVDC system design, losses, availability, cost, manufacturing, installation and testing. The discussion finds that the cable unavailability as well as the impact of voltage drop beyond commercially available transformer tap ranges can limit the technical and feasible length to between 3000- and 7000 km, depending on the cable design and quality. It concludes with a reminder to ensure that a detailed availability sensitivity study that adequately explores all factors that affect cable failure and repair should be included in the business case prior to any investment decision is made.