Why a predictable cold snap crippled the Texas power grid

As Texans cranked up their heaters early Monday to combat plunging temperatures, a record surge of electricity demand set off a disastrous chain reaction in the state's power grid.

Wind turbines in the state's northern Panhandle locked up. Natural gas plants shut down when frozen pipes and components shut off fuel flow. A South Texas nuclear reactor went dark after a five-foot section of uninsulated pipe seized up. Power outages quickly spread statewide leaving millions shivering in their homes for days, with deadly consequences.

It could have been far worse: Before dawn on Monday, the state's grid operator was "seconds and minutes" away from an uncontrolled blackout for its 26 million customers, its CEO has said. Such a collapse occurs when operators lose the ability to manage the crisis through rolling blackouts; in such cases, it can take weeks or months to fully restore power to customers.

Monday was one of the state's coldest days in more than a century - but the unprecedented power crisis was hardly unpredictable after Texas had experienced a similar, though less severe, disruption during a 2011 cold snap. Still, Texas power producers failed to adequately winter-proof their systems. And the state's grid operator underestimated its need for reserve power capacity before the crisis, then moved too slowly to tell utilities to institute rolling blackouts to protect against a grid meltdown, energy analysts, traders and economists said.

Early signs of trouble came long before the forced outages. Two days earlier, for example, the grid suddenly lost 539 megawatts (MW) of power, or enough electricity for nearly 108,000 homes, according to operational messages disclosed by the state's primary grid operator, the Electric Reliability Council of Texas (ERCOT).

The crisis stemmed from a unique confluence of weaknesses in the state's power system. Texas is the only state in the continental United States with an independent and isolated grid.

That allows the state to avoid federal regulation - but also severely limits its ability to draw emergency power from other grids. ERCOT also operates the only major U.S. grid that does not have a capacity market - a system that provides payments to operators to be on standby to supply power during severe weather events.

After more than 3 million ERCOT customers lost power in a February 2011 freeze, federal regulators recommended that ERCOT prepare for winter with the same urgency as it does the peak summer season. They also said that, while ERCOT's reserve power capacity looked good on paper, it did not take into account that many generation units could get knocked offline by freezing weather. "There were prior severe cold weather events in the Southwest in 1983, 1989, 2003, 2006, 2008, and 2010," Federal Energy Regulatory Commission and North American Electric Reliability Corp staff summarized after investigating the state's 2011 rolling blackouts. "Extensive generator failures overwhelmed ERCOT's reserves, which eventually dropped below the level of safe operation."

ERCOT spokeswoman Leslie Sopko did not comment in detail about the causes of the power crisis but said the grid's leadership plans to re-evaluate the assumptions that go into its forecasts. The freeze was easy to see coming, said Jay Apt, co-director of the Carnegie Mellon Electricity Industry Center.

"When I read that this was a black-swan event, I just have to wonder whether the folks who are saying that have been in this business long enough that they forgot everything, or just came into it," Apt said. "People need to recognize that this sort of weather is pretty common." This week's cold snap left 4.5 million ERCOT customers without power. More than 14.5 million Texans endured a related water-supply crisis as pipes froze and burst. About 65,000 customers remained without power as of Saturday afternoon, even as temperatures started to rise, according to website PowerOutage.US.

State health officials have linked more than two dozen deaths to the power crisis. Some died from hypothermia or possible carbon monoxide poisoning caused by portable generators running in basements and garages without enough ventilation. Officials say they suspect the death count will rise as more bodies are discovered.

In the central Texas city of Austin, the state capital, the minimum February temperature usually falls between 42 and 48 degrees Fahrenheit (5 to 9 degrees Celsius). This past week, temperatures fell as low as 6 degrees Fahrenheit (-14 degrees Celsius).

In November, ERCOT assured that the grid was prepared to handle such a dire scenario.