On 12 September, the U.S. Department of Energy released a new timeline of the 14 August blackout, which was prepared by a U.S.-Canadian expert team. It is significantly more detailed and complex than the initial timeline that was prepared by the North American Electricity Council and reproduced by many publications in a number of versions.
The task force cautioned that its report does not presume to explain why the blackout occurred or to "assign fault or culpability." That will be the job of an investigative team managed by the task force.
That said, the new event sequence, even more than the earlier timelines, highlights the critical situation of northeastern Ohio as an electricity choke point, both into Michigan and the upper Midwest and to New York and Pennsylvania to the east.
The failure of many lines and the self-protective tripping of some generators led to a situation at 4:10 Eastern Standard Time in which the eastern Michigan and northern Ohio systems had little available generation left, while voltage was declining throughout the areas. Then, at 4:10:38, when the main transmission line running east from northeast Ohio failed, power that had been flowing west into Ohio reversed direction. That surge started to literally flood grids around Lake Erie through northwest New York State and Ontario, as power sought to find its way into northern Michigan to satisfy unmet load in that region.
With the transmission system to the east and northeast of Ohio now getting severely overloaded, the northeast system proceeded to unravel at a breathtaking pace. Within six seconds of the 4:10:38 failure, the Pennsylvania system disconnected, and before a minute was out, New England had unlinked itself as well. By 4:13 p.m., the cascading sequence was "essentially complete," the report concluded.
The complete text of the event sequence released by the task force is at: https://www.oe.energy.gov/documents/1282003113351_Blackout_Summary.PDF