[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.