What seems to have happened:
- One large power plant went offline (probably nuclear)
- Voltage control via spinning turbines didn’t happen, and voltage was significantly higher than it should have been.
- All the solar power facilities had the same threshold for overvoltage shutdown, so they all turned off at once.
Better planning for voltage regulation and having different solar power facilities have different trigger points for shutting down in the face over too-high grid voltage would prevent this.