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.
At one site in a jungle, my employer had an electrically driven flywheel built that ran on that country's mains grid, paired with an AC alternator on the flywheel that fed the building. This solved a variety of frequency and voltage problems, including mains outages. If the mains went dark, a diesel generator would kick in to boost the flywheel RPMs on an as-needed basis rather than constantly run.