In article , Richard
Catlow writes
Strictly speaking Dinorwig was not designed as a backup generator, nor
is used as one, as it is powered from pumped hydro storage and thus
has a finite time from which it can provide useful energy conversion
before the head of water becomes exhausted.

These are all characteristics of a UPS.

I'm not sure how important it is to make a distinction between:

Generating capacity failing to cope because a power plant has
tripped

and

Generating capacity failing to cope because demand has step-
function increased.

The first is very much a "backup", but the second (without an identical
extra source of supply) would have caused the grid to fail.

Over to you.
--
"now, the thing you type on and the window you stare out of are the same thing"