On August 14, 2003, at least 50 million people lost the ability to cool their homes, refrigerate food, light offices, compute and commute, along with the myriad other necessities electricity provides in the modern world. A failed power line in Ohio set off a cascade of events that triggered the largest blackout in North American history and crippled much of the northeastern U.S. for two days.
In the year following the disaster the U.S.–Canada Power System Outage Task Force convened to figure out the causes of what happened.
Just prior to the 10th anniversary Scientific American spoke with electrical engineer Jeff Dagle, a member of the task force and a specialist in power-grid resilience at Pacific Northwest National Laboratory, to find out what we know now that we didn’t then, and whether similar mishaps could still happen.
[An edited transcript of the interview follows.]
What happened on August 14, 2003?
The blackout itself, which was a big one, affected 50 million people and 60,000 megawatts with an estimated economic impact of $10 billion. It started at 3:05 P.M. on August 14. A power line tripped [went offline] in northern Ohio. It was actually carrying less [electricity] than it was capable of so it should not have tripped, but trees under the line had gotten too close to the wire. It's just energized aluminum suspended in a wire and it relies on the air to provide insulation. If something gets too close it will arc and short-circuit.
So we lost a 345-kilovolt line in northern Ohio. Normally the grid is designed to have enough resilience built into it that losing a single line doesn't have any impact. But on that day there was also a problem with the software in the control center. The utility [First Energy] that owns that line would normally be looking and taking preventative action, but they didn't even know the line had tripped.
Because of the loss of that first line, about 30 minutes later an adjacent line tripped due to a similar cause. As with any heavily loaded transmission line, the heat from the current heats the metal and it expands. The line sags down closer to the things below it. So it was within its rating but trees [again] had been allowed to grow too tall. Fifteen minutes later a third line tripped.
Now we have three key lines over a period of about 45 minutes that tripped offline. The grid isn't designed for that level of redundancy.
At this point, the power is still trying to flow. So you get this cascading sequence of events that picked up speed. Shortly after 4 P.M. this cascade progresses outside of northern Ohio. So it blacks out Akron and Cleveland and then works its way around to Detroit and works around Lake Erie, taking out Toronto. Then it works around to the northwest and much of New York State trips off along with a big chunk of Ontario, making it the largest blackout ever in North America.
You were on the committee that investigated the event. Were the trees the main problem?
Another key root cause was this loss of situational awareness, which went a little deeper than just a software glitch. We were curious why the operations center didn't start to put the clues together. In fact, it wasn't until the lights went out in the control room that they really understood the grid was in peril.
They were getting a lot of phone calls and activity suggesting that there was a problem but they didn't connect the dots. They allowed the system to fail over that span of about an hour.