Get the latest tech news
Spain is about to face the challenge of a "black start"
Major power facilities require power to operate, and there’s lots of unmet demand.
Facilities that boil water have lots of powered pumps and valves, coal plants need to pulverize the fuel and move it to where it's burned, etc. You can think of each bit of power-using hardware as a source of friction that would slow the frequency down, while each generator can potentially speed the grid back up. Beyond that, both countries have invested heavily in renewables, with Portugal supplying about half of its power from wind and hydro, having closed its last coal plant in 2021.
Or read this on Hacker News