[Armisael16]

You only need to match phase at the connection point, so the phase of the grid in France doesn't really matter (as long as ENTSO-E isn't destroying itself before you do anything).

This does get much trickier if you have multiple connection points and it is impossible to be synchronized at all of them because the in-grid distance between the two points is different on the two grids. Then you definitely do need fancier tricks.

[tremon]

True, but you need to have a very large unbalanced grid for that to occur: at the grid frequency (50 Hz), the corresponding wavelength is 6000 kilometer. So assuming that math is correct, you would need a path length difference of 3000km between two transformers to get full phase inversion between the paths.

That's more than twice the max diameter of the entire European grid (Tunesia-Gibraltar-Turkey seems the longest path). So as long as there's transformers every few hundred kilometer, and multiple paths between most of the connection points, the overall phase synchronization should be adequate to not need those fancier tricks.