The reason AC is often believed to be more efficient is because higher voltages of either type are more efficient (less loss to electrical resistance), and it's much easier to step voltage up and down using AC.
Transformers are very efficient and easy to
make for sure, and can ‘easily’ step up to millions of volts.
Longer distance runs of AC can have a lot of losses though due to inductance, especially when they’re close to large conductive masses (like seawater or earth).
DC requires expensive conversion equipment to get back to AC for the grid.
AC is cheap for short runs. Usually AC vs DC high voltage starts to be equally cost effective around the several hundred miles of buried lines, and a clear winner around the 1000 mile range (efficiency and capex wise).
HVDC can also allow grid balancing when disparate grids are connected (different frequencies and voltages for instance).
Longer distance runs of AC can have a lot of losses though due to inductance, especially when they’re close to large conductive masses (like seawater or earth).
DC requires expensive conversion equipment to get back to AC for the grid.
AC is cheap for short runs. Usually AC vs DC high voltage starts to be equally cost effective around the several hundred miles of buried lines, and a clear winner around the 1000 mile range (efficiency and capex wise).
HVDC can also allow grid balancing when disparate grids are connected (different frequencies and voltages for instance).