Not all power plants can self-feed the power they need to keep generators working. Often this is a diesel generator on the plant because the only other source o

Not all power plants can self-feed the power they need to keep generators working. Often this is a diesel generator on the plant because the only other source of power is 20MW generators that simply can't be tuned down enough to handle the few kilowatts it takes to keep the generators turning. Almost all plants will trip from the grid if it gets bad and remain idling for some time. But if that happens, the grid is still dark and the black start generators are the best equipped to bootstrap the grid. You can't start with the big generators too, so you can initially only bring in small plants until the grid capacity is big enough to not trip out when the big generators come online in a wrong way. There is a good chance that by the time the grid is stable, atleast some power plants will have had to shutdown entirely and spin back up from a black start with only external power.