Comment by _heimdall
4 hours ago
The biggest challenge is keeping the energy almost perfectly in phase. US power runs at 60Hz and most grid equipment fails quickly if that changes even a little bit.
If I'm not mistaken, that was what hit Spain when much of their grid went down for hours. The analysis I heard was that they were running entirely on renewables and it fell out of phase causing large scale safety shutdowns.
Fortunately, interconnects cannot go out of phase easily. If one region is out of phase with another, massive currents flow down the transmission lines within milliseconds, causing generators at one side or the other to slow down or speed up. I have seen this effect live at a hydroelectric dam. We were watching the turbine shaft spin at the moment they switched on some giant interconnect. There was a phase-alignment dial and it just instantly pinged up to 0 while the spinning shaft shuddered, slightly altering its rate of rotation in the blink of an eye.
That's the usual failure mode of any grid, aggressive load shedding will be used as a last resort to keep the frequency within tolerances.
The renewable aspect is because historically they were installed with "grid following" inverter systems instead of "grid forming" which can maintain frequency even if temporarily islanded in a partition without spinning mass generators to set frequency.
There's nothing intractable about a 100% renewable grid, the technology exists and has already been deployed in new installations.