Cato’s Still Groping in the Dark? summarizes last year’s blackout:
In short, three 345-kilovolt transmission lines went down when heat caused them to sag and come into contact with trees. That created an imbalance between supply and demand along the lines feeding the Cleveland area, which led, in turn, to higher current flow and accompanying lower voltage on a large portion of the remaining Eastern interconnection as the power raced along other routes to get to Cleveland.When devices known as ‘relays’ detected the unusual power flows around the Cleveland area, they automatically triggered circuit breakers that removed a number of lines from service (a preventative measure to ensure that billions of dollars of capital stock are not fried by unusual power flows). In the words of the report, the ‘cascade became a race between the power surges and the relays.’ The lines that tripped first were generally the longer lines that split the grid into those sections that blacked out and those that recovered without furthering the cascade. The upshot is that ‘protective relay settings on transmission lines operated as they were designed and set to behave on August 14.’