10% of demand is about double that... Dinorwic couldn't hold the frequency up on it's own and the automatic frequency trips would activate knocking even more capacity off the grid and you end up in a cascade to black, or isolated islands, *very* rapidly.
Remember this is losing 10% without warning, the first thing you'd know would be alarm(s) that station(s) X, (Y, Z) have dropped off line, followed very rapidly by low frequency trips as the automatics try to protect the grid.
Dinorwic can come online flat out in 11 seconds or so, if it's spinning and synchonised. If it's not it's over a minute that's far to long once the grid frequency starts to drop.
It was the low frequency trips that gave the blackouts when Longannet and Sizewell B decided they didn't want to play anymore.
Yes, there is plenty of capacity buried away but not much that is
*instantly* available. It's the first few seconds that are critical, once the automatics have stopped tripping you can then look at the mess you have left. What is still connected to what, which stations are still online, which can be bought back quickly, which will need a day or to to "reset" having had to emergency vent their boilers and dump the fires. 1.5 GW gave it quite a head ache for quite a while.I agree that the costs of have dual capacity for wind/solar is stupid what I'm picking up on is the last part of: "Any other event that takes out more than one power station unexpectdely is likely to be massive enough so that the loss of 10% of our supply will be the least of our worries."
The only worry when two power stations went unexpectdely, with the loss of much less than 10%, was "will the grid stay up".