But what caused the frequency drop? Large-scale grids are designed and operated in such a manner that any single fault, even one which causes a frequency drop (like a generator or a power line getting disconnected), will not cause a blackout. Which means: if there isn't enough inertia to compensate the frequency drop caused by a single fault anywhere in the grid, the system operator will either order photovoltaics and wind turbines to reduce their generation to a safer level, or order traditional rotating generators to operate as synchronous condensers (which adds inertia without adding generation).

Which means that either there was a double fault (two faults close enough in time that there wasn't enough time to reconfigure the system to a safer state before the second fault), or that the modeling of how the photovoltaics and wind turbines would react to a single fault was incorrect (for instance, expecting them to stay connected for longer on that level of frequency drop). My personal guess is that we're going to see a repeat of what happened here in Brazil in 2023, as I explained in another comment on an earlier thread (https://news.ycombinator.com/item?id=43821801), where a single fault was enough to destabilize the system because the inverters in wind and solar power plants disconnected earlier than expected.