Here's the more advanced version.[2] This discusses the Carrington event, the Hydro Quebec event of March 13, 1989, and what PJM does when there's a problem.
The effect of a geomagnetic disturbance is that DC currents appear between distant points that are earth-grounded. This is caused by the ionized solar wind flowing through the space between power lines and the ground. Current flow through a loop generates voltage, remember. That puts DC currents into AC transformers, which can cause partial magnetic saturation, which makes the coils a resistive load for part of the cycle, which causes heating. Transformer damage is possible if power levels are not reduced. That happened to two transformers in 1989. The DC currents aren't huge; 10 amps is enough to cause trouble.
So, PJM now has DC ground current meters at a few key points. These report to the system control centers (they have two, just in case), where operators can take actions to reduce power flows at the vulnerable points. In [2], there's considerable detail about this. ("The procedure is implemented when a DC measurement of 10 amps or greater is detected at either Missouri Ave or Meadowbrook. Reduce Salem 1 & 2 units to 80% power, and Hope Creek to 85% power ...")
Journalists and pundits writing on this subject should read that before writing. If you don't understand it, find and take the "PJM 101" online training course. PJM's materials are written for the people who run the system, and there's little nonsense or hype.
The reports go out to anyone who gets PJM alerts. Those have been on the web for years. Since the last time I looked at this, they've added apps for IOS and Android. Since energy traders need this info, PJM lets anybody view it.
[1] https://insidelines.pjm.com/geomagnetic-disturbance-what-is-...
[2] https://pjm.com/-/media/training/nerc-certifications/trans-e...