The chief executive of ERCOT, the Texas grid operator, said that "extremely cold" temperatures and windy conditions caused valves, pipes and other equipment in some power plants to fail, which in turn triggered the need for rolling blackouts across Texas yesterday.

"There's really no pattern that we're observing," said Trip Doggett, in a press conference this afternoon. "It was spread across the state. No particular area of the state; no particular owner of the generation. It included units that were online that tripped offline. It also included units that were attempting to come online that were unsuccessful."

ERCOT has said that the power emergency is now over, but Doggett said that conservation remains critical, and South Texas remains vulnerable.

"We continue to have plant outages in the Valley that concern us," Doggett said.

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Yesterday's events "unfolded very quickly," Doggett said. More than 50 power plants, representing over 7,000 Megawatts of generation, went offline, he said. Today, more than 3,000 Megawatts remain offline.

Mexico, Doggett said, supplied power to Texas yesterday and this morning. It's particularly cold in Mexico today, and Texas is unable to reciprocate because of the state's tight grid situation. Doggett would not comment on whether he had received a request for assistance. Texas has its own grid, separate from the rest of the United States, but does have a few ties, particularly to Mexico.

Doggett said that ERCOT was "preparing a list today of the specific resources that [had] outages, and the generation types and a description of the problems that created the outage." He said he planned to share that with lawmakers; ERCOT has balked at releasing this type of list to the press.

"There is competitive information that can be gleaned if their competitors are aware that they are out of service," Doggett said.

Callers into the press conference were in a listen-only mode, so additional questions remain to be answered.

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