As much of the U.S. braced for more harsh winter weather and extended power outages, utilities and grid operators were working to restore service to customers without electricity in the grip of a fierce Arctic blast.

Grid operators across the U.S. shed loads amid a bitter cold snap that triggered widespread power outages and emergency measures to limit and prevent them. As of midday Feb. 16, nearly 5 million customer accounts were without power across the U.S., according to poweroutage.us, which aggregates U.S. utility outage data.

The Federal Energy Regulatory Commission and the North American Electric Reliability Corp. said Feb. 16 that they would open a joint inquiry into the event.

In Texas, the hardest-hit state, more than 4 million customers were still without power Tuesday morning as the Electric Reliability Council Of Texas Inc. and the state's utilities struggled to bring more lost generation back online. Late in the day Feb. 15, ERCOT reported that power had been restored to 500,000 households. Another 400,000 had been restored by midday Feb. 16, the grid operator said.

Weather and added generation outages brought the load shed in ERCOT's territory to 18,500 MW overnight Feb. 15-16. Even as the grid operator said more customers would see power restored the afternoon of Feb. 16, another winter storm threatened to bring additional snow and freezing temperatures to much of the state, with a winter storm warning in effect through Feb. 18.

"The number of controlled outages we have to do remains high," said Dan Woodfin, senior director of system operations, on the official ERCOT Twitter account early Feb. 16. "We are optimistic that we will be able to reduce the number throughout the day."

Governor promises 'reform'

Texas Gov. Greg Abbott, a Republican, said he would make "reform" of ERCOT an emergency priority in the current legislative session, calling for an investigation of the grid operator to "ensure Texans never again experience power outages on the scale they have seen over the past several days."

"The Electric Reliability Council of Texas has been anything but reliable over the past 48 hours," Abbott said in a statement. "Far too many Texans are without power and heat for their homes as our state faces freezing temperatures and severe winter weather. This is unacceptable."

The culprits in this week's outages were widespread, but the largest contributor in ERCOT was the failure of natural gas-fired power plants to produce power as expected. Low production from wind farms and the outage of at least one nuclear unit also contributed.

"Total wind output is slightly below expectations, but the main supply issue is lack of available thermal generation (both gas and coal) due to freezing conditions" in Texas, consulting firm ICF International Inc. said. Gas production dropped at least 16% because of well freeze-offs and shutdowns of processing plants, ICF said. ERCOT's extreme peak load scenario anticipated wintertime demand of up to 67.5 GW, but the day-ahead forecast for 8 a.m. Feb. 15 was 74.5 GW.

The "magnitude of the forecast error was massive," ICF said.

At least in part, the "cold dome" freezing much of the country can be attributed to climate change as warmer Arctic air weakens the jet stream, sending blasts of frigid air southward, said Joshua Rhodes, a research fellow at the Webber Energy Group at the University of Texas at Austin and a founding partner of IdeaSmiths LLC.

Controlled interruptions

The Southwest Power Pool said Feb. 15 that electricity use exceeded generation in its 14-state balancing authority area. The grid operator declared an emergency alert just after 10 a.m. CT and instructed transmission system operators to reduce demand while utilities determined how to curtail use.

"In our history as a grid operator, this is an unprecedented event and marks the first time SPP has ever had to call for controlled interruptions of service," Lanny Nickell, SPP's executive vice president and COO, said in a statement. "It's a last resort that we understand puts a burden on our member utilities and the customers they serve, but it's a step we're consciously taking to prevent circumstances from getting worse, which could result in uncontrolled outages of even greater magnitude."

The Midcontinent ISO said in a Feb. 15 statement, "Sustained frigid temperatures and winter weather impacting the Midcontinent Independent System Operator (MISO) South Region contributed to the loss of generation and transmission."

"In the days ahead, we will examine the root causes of these events and what we can do to prevent them in the future," FERC Chairman Richard Glick said in a statement. "But, for now, all our efforts must go toward restoring power during these incredibly challenging times."