Powerful damage by the numbers
The 2011 tornadoes that ripped through the Tennessee Valley toppled transmission lines and cut off power across TVA’s 7-state region:* 847,311 homes and businesses lost power* 108 transmission lines were out of service, including 15 of TVA’s high-power 500-kV transmission lines* 75 municipalities or power coops suffered power disconnections from TVA* $39 million in repairs were required to replace and repair damaged TVA equipment after the storm* 95 percent of affected customer points were reconnected within a week, although only a fourth of damaged structures were repaired in the first week. Some took more than a month to replace or repair.Source: Tennessee Valley Authority
The red lights that dot the giant wall inside TVA's power control center quickly turned green on the morning of April 27, 2011, signaling the loss of generation as tornadoes ripped across Mississippi and Alabama, twisting dozens of TVA transmission lines like pretzels in the wind.
"Looking at the map and the plethora of alarms coming in for the operators, it was overwhelming," recalled Chris Thompson, senior transmission operator who was on duty that day. "To say it was a daunting task to put it back together again is a vast understatement."
The worst tornadic storms to ever hit TVA's seven-state region downed some or all of 108 transmission lines, leaving nearly 850,000 customers in the dark and cutting off all off-site power to TVA's biggest nuclear power plant for the only time in its 44-year history. Tornadoes cut off power for several hours to TVA's Browns Ferry Nuclear Power Plant, forcing operators to shut down all three of the reactors that collectively supply 10 percent of all of TVA's power.
TVA managed to restore power to 95 percent of those who lost power within a week of the 2011 storms. But the complete recovery took 65 days - one of the longest recovery periods in modern times for TVA, which prides itself on better than 99.999 percent reliability for its power delivery.
Five years after the 2011 storms, TVA is still assessing how it could do better if such storms hit the valley again.
TVA officials acknowledge Mother Nature is often more powerful than their own networks, regardless of how hardened they become. But the federal utility has added more training and shelters, both to help protect crews and to reduce the length of power outages.
TVA also has boosted operator training for storms and limited where vegetation may grow close to transmission lines to better protect electric lines from falling trees.
"As far as being better prepared today, well, practice makes perfect and, unfortunately, that was great practice," Thompson said. "I think we would do better if it happens again, but we hope it never does."
T. Patrick Walshe, senior manager of resource operations and analysis and a meteorologist at TVA who worked as a weather forecaster when the 2011 tornado struck the valley, said he and others foresaw stormy weather nearly two days in advance. But no one predicted it would end up being as bad as it was.
"It was unlike anything I had ever seen in my life, and I have been doing meteorology for a long time," he said.
The 2011 storm hit in the same week just a year after Nashville suffered severe floods. So Walshe said the initial weather concerns were for heavy rains and floods in the valley.
But within a couple of days of the storm, Walshe said he recognized the potential for storms across the region also included the potential for high winds or tornadoes and he began warning TVA facilities and distributors about the potential for problems. On the morning of April 27, tornado warnings began about 9 a.m. CST and continued off and on throughout most of the day.
Even TVA's own employees in the Chattanooga Office Complex had to go into the basement of the downtown office complex when the tornado warnings extended to downtown Chattanooga.
"Everybody above the ground floor was down here for six and a half hours that day," Walshe recalled. "So it was a crazy day as far as how our operations go."
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The early morning tornadoes did only minor damage, but by the afternoon, major tornadic cells formed and spawned twisters in Alabama and Mississippi that wreaked havoc on much of TVA's transmission system in those areas.
The National Weather Service radar station at Hytop, Ala., was knocked out of service, "but we could still see all these cells, and the atmosphere was just so right for tornadoes," Walshe said.
TVA was about as ready for the storm as it could be, but in the end, Mother Nature had its way, toppling giant transmission towers when at least three EF5 tornadoes struck the TVA service territory.
TVA officials say they continue to look for ways to better protect its power plants and transmission lines.
A major change has come from regulations adopted by the Federal Emergency Management Agency for all utilities in the wake of the 2003 Northeast blackout. In response, TVA decided in 2012 to cut any tree that is or can ever grow taller than 15 feet in its 100-foot, 150-foot and 200-foot rights of way near its transmission lines.
Although many neighboring property owners have objected to TVA's vegetation control measures, utility officials insist that limiting where trees are located near power lines helps reduce the chances of electric lines being toppled or damaged.
"Managing vegetation and maintaining the right of way for transmission lines not only aids storm recovery, but also helps prevent system damage during the storm," said Jason Regg, senior manager of TVA's Right of Way Program. "When trees infringe on the right of way, they often end up falling on the power lines. In a tornado, you often see just the opposite: the lines get ripped down and tangled in the trees. Our vegetation maintenance program is vital in helping address both of these issues."
Most power outages - and some of those in the 2011 storms - are caused by lightning strikes on power lines. TVA designs its lines to withstand surges of up to 80 kilo-amps in order to minimize customer impact from severe storms.
TVA uses a variety of grounding systems and arrestors to help harden its power grid against lightning strikes, which normally account for at least half of all of its power outages.
TVA also participates in studies by the Electric Power Research Institute to audit customers' facilities, processes, equipment, automation, controls and other components in order to make plant processes more resilient to voltage problems, including events from tornadoes.
"We can stage crews and have supplies ready, but there wasn't much more we could have done to get ready," Walshe said. "We'll continue to try to do whatever we can to be ready for storms, but you can't prepare for all of the types of tornado damage we saw that day."
Because TVA restored service to nearly all customers within a week, even with only 25 percent of its structures repaired following the storms, most customers were understanding of the power outage, Thompson said.
Customer satisfaction surveys conducted by TVA showed overall satisfaction actually increased among TVA users in 2011.
"Despite the damages across the system, our customer satisfaction actually went up in 2011," said Theo Laughner, senior program manager for power quality. "Customers understood it was an extremely unusual and challenging event, and they knew we were working as hard as we could and appreciated successful efforts to get things back up and running."
Contact staff writer Dave Flessner at email@example.com or at 423-757-6340.