DC is always more efficent to transfer. But it was difficult to efficiently convert to high voltage and back so you only did it when necessary. The reason unde

DC is always more efficent to transfer. But it was difficult to efficiently convert to high voltage and back so you only did it when necessary. The reason underwater links (eg France-UK) were commonly DC is that they linked countries whose grid wasn't synchronised - the same reason long distance grids in the US are DC