Fox is saying that 5 nuclear substations went out and that now 2 are back up, they do not know what the cause was.
Katherine Heritage said that Homeland Security is keeping a
close eye on the situation to see if it was some sort of
sabotage.
Most, if not all, major power plants (nuclear and otherwise) have mechanism to prevent overloading and associated consequences if a substation fails or similar. This can create a cascade effect causing power plants to go off-line to prevent burnouts, etc of the equipment. Source of the incident would be most interesting because if it was intentional (terror related), it could allow other chaos to erupt if done on a larger scale/level.