[philipkglass]

According to the operator report linked in another comment by leymed [1], the problem was not a lack of spinning mass (inertia) but voltage instability. From page 16 of the PDF:

The incident was NOT caused by a lack of system inertia. Rather, it was triggered by a voltage issue and the cascading disconnection of renewable generation plants, as previously indicated. Higher inertia would have only resulted in a slightly slower frequency decline. However, due to the massive generation loss caused by voltage instability, the system would still have been unrecoverable.

[1] https://news.ycombinator.com/item?id=44360052

[baq]

Obviously I’m as good of a grid operator as I was a stealth bomber expert on the weekend, but superficially that just doesn’t seem right. Maybe I’m underestimating how much spinning mass would be required, but that still qualifies as ‘not enough was present’.

[rcxdude]

You very much are underestimating it. Spinning mass helps even out very short term fluctuations in supply vs demand. Like on the timescale of tens of seconds, even when the whole grid is spinning mass. Even 10x the inertia in the grid would have maybe bought a few extra minutes, because the problem by the point the grid was collapsing was there were not enough plants online to provide the demand.

(Spinning mass on its own doesn't do much to deal with the voltage fluctuations. It's entirely something that's reactive to grid frequency, which is the most 'global' indicator of supply vs demand in a grid, since it can't fluctuate locally. But voltage and current can vary wildly in different parts of the grid, and required separate management)

[eldaisfish]

please can you explain what doesn't seem right?

[baq]

They admit they had plants offline when they should be available and then they say it wouldn't matter and the grid would've collapsed anyway..? Either they're in a very very bad spot and stop short of saying it outright in the report or it would've averted the disaster if the not-spinning mass was spinning as it should have.

[eldaisfish]

rotational inertia has nothing to do with voltage control.