[thackney]

However, what would be the secondary effect if all power went out in an area from Buffalo to Chicago?

It would be dark, for a while, if it happened at night. Likely some areas into the next night.

Would the rest of the U.S. Power grid try to compensate for the loss, taxing the grid to the point where other sites start to collapse, with a domino effect of taking the whole system down?

If it went down, there is no load to pick up. The connecting lines are limited to their capacity, the relays are not going to let too great a power pass. If the protective relays are not working, the line is down and no power passes.

Or would the grid have the intelligence to automatically break off the parts of the country that drop, preserving the integrity outside the primary area?

Yes, the dark areas would be limited. Possible to be large for a while, but not unlimited.

[Tilted Irish Kilt]

Whether a Carrington Event occurring , or a singular nuclear blast 60 miles overhead
or even multiple warheads, or three separate attacks on West Coast, the Heartland, and the East Coast would have an effect on the electrical distribution .
As I recall it was last December that we learned of an attack on a sub-station on FreeReport :
"The Complex 'Military-Style' Raid on California Power Station Spooks U.S.,Saturday, December 28, 2013"
and the FED kept it quiet from 1 AM attack on April 16 , 2013 until the event was publicized on 12/28/13 .
I believe that a Carrington Effect or nuclear EMP would be devastating to fuel production , food production and distribution, transportation ,
, water purification and distribution ,all electronics and communications.
The fact that this incident at the PG&E Metcalf power substation, southeast of San Jose was almost successful in creating a cascading power failure
is of concern , just as much as the attack being kept quiet for 8 (eight) months by the government .
Fear mongering ,.. no , ..I doubt it !