Hybrid transmission systems ( HVDC+ FACTS) offer many technical and reliability advantages. There are now ways of transmitting 3-4 GW over large distances with only bipolar DC transmission system. This is usefull in offshore wind projects.
HVDC provides reduction in transmission costs due to less losses, and for sea cable transmission with distances over 80 km, DC is the only technically feasible solution.
Today's maximum rating of 6 GW ( at DC voltages of =/- 800 kv) - with UHV AC at voltages of 1000 KV, power transmission on one 3-phase circuit with 10 GW is feasible.
HVDC at present are limited to 600 KV, in order to enable bipolar ratings of 6 GW, we need 750 KV HVDC.