There's by now a lot of DC links in Europe for example going from the European continental grid to the Nordic grid. Denmark is split between the two so it has a country internal DC link too.
There are several DC links in Canada too, i.e. the underwater link between the Lower Mainland of British Columbia and Vancouver Island, which uses only one conductor and the water itself as the other. The North American grid is also DC linked betwen Alberta and Saskatchewan at one point.
DC links solve a myriad of synchronization problems.
