HVDC has a very important role to play in transmission networks and is an economical viable option at ultra-high voltages (800 kV) in several regions in the USA. These DC links need to operate as hybrid HVAC and HVDC networks for integrating and controlling remote renewable resources, and they are well established to interconnect asynchronous areas. At the relative low capacity factors of wind (40%) and solar PV (20%) power production, HVAC networks are not utilized at an economic level for integrating these intermittent resources to the load centers, especially over long distances or with submarine cables from a relative weak remote AC network. HVDC is clearly the better option, especially the newer voltage source converter technologies. For no or very limited additional cost, two STATCOMs are added on both sides of the HVDC link. These STATCOMs are excellent to interconnect the remote power from relative weak networks to the load centers.
Multi-terminal HVDC networks and hybrid HVDC imbedded into HVAC networks are already commercialized and implemented in Europe and will provide the additional capacity to the transmission network in the USA. Furthermore these hybrid networks will at the same time increase the transmission capacity with reduced right-of-way (ROW) requirements. A HVDC link can transmit three to five times the power in the same ROW when compared to HVAC networks. At the same time, financing these new HVDC links is easier than traditional HVAC lines under merchant line agreements, since full control of the power flows can be guaranteed between remote resource and load center.
It is not a question of AC or DC but both, especially in the high-voltage and ultra-high-voltage transmission networks.