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HOUSTON -- Frozen valves, control equipment and transducers at Texas largest power plants caused emergency rolling outages on some of the coldest days of the year, and it remains unclear whether plant operators properly prepared and maintained their facilities for winter, state power regulators said Thursday.

The Electric Reliability Council of Texas cancelled the emergency procedures on Thursday after recovering more than half of the 8,000 megawatts of electricity lost a day earlier. But ERCOT called on residents to continue conserving power as 3,000 megawatts remain offline while frigid temperatures and the threat of ice and snow lingers over Houston and Southeast Texas.

The equipment that froze should have been insulated as part of the power plants preparation for winter, ERCOT s CEO Trip Doggett said. ERCOT and the Public Utility Commission are looking into whether insulation at the 50 plants that failed was done properly or whether it was inadequate for the unusually frigid temperatures.

We have to let the owners of the generators speak to their issue . usually these plants go through a winterization process in preparation for cold weather to ensure heat and insulation is wrapped around, pipes, valves and other equipment, Doggett told reporters. So it s only speculating why that wasn t adequate in this case.

Clara Tuma, a spokeswoman for the Lower Colorado River Authority that operates five power-generating plants, said they prepared for the cold, bringing in heaters for the units and installing tarps to protect them from wind. Valves were opened to keep water flowing and avoid freezing pipes, she said. She declined to say whether any units had failed, but said they had the same issues as every other utility provider.

This week, we knew the weather was chilly and was getting colder, so we were in an all hands on deck situation, Tuma said.

Things were so dire Wednesday, Mexico exported power across the border, Doggett said. By Thursday, however, Mexico was struggling with its own cold weather and could no longer help.

By Thursday afternoon, Luminant, which had four units fail Wednesday, had three generating power again, the company said in a statement.

But while the emergency is over for now, Doggett warned the rare freezing temperatures plaguing Southeast Texas meant residents need to conserve energy until temperatures moderate over the weekend.

Our optimism is tempered by concern, especially in South Texas, Doggett said. It s very cold in the valley today, much colder than normal, and we continue to have plant outages in the valley that concern us.

Still, questions about what caused three-quarters of the state to suffer several controlled outages remained unanswered, and Doggett refused to take questions from reporters who dialed into a news conference held in Austin on Thursday. It was the first time the council answered media queries, more than 30 hours after the first outage at 5:09 a.m. Wednesday.

Doggett declined to list the failing plants, saying it could harm competition in a state that has deregulated electricity distribution and many consumers can choose a provider.

Already, he said, ERCOT is preparing for the summer when oppressive heat creates transmission line problems, Doggett said.

Terry Hadley, spokesman for the Public Utility Commission, said it remains unclear whether the insulation was improper or if there was a problem with the integrity of the pipes. At the moment, there are expectations that the plant operators will prepare for extreme weather, but after this incident there could be more regulation put in place. Similar issues are rare in colder areas of the country, partly because they deal with such temperatures all the time and are better prepared, he said.

But that doesn t excuse not being able to operate when an unusual cold event happens in Texas, Hadley said.

Officials have found no pattern and no area of the state or operator that was more or less prone to issues, Doggett said.

It included units that were online that tripped offline and it also included units that were attempting to come online that were unsuccessful, he said, explaining how the grid fell below the 2,300 megawatts of reserves, triggering the emergency.

The outages, Doggett said, are the last step taken to ensure the grid s security is not compromised a situation that could cause statewide blackouts. The grid was never close to going down.

We were very secure, he added.

Meanwhile, the excitement and anticipation in Houston were

palpable as a town more accustomed to heat, humidity and hurricanes prepared for snow. The Houston Independent School District cancelled school on Friday, giving students a rare snow day before a flake had fallen.

State and local transportation officials prepped rarely used salt trucks and snow plows to clear Houston s massive freeway system. And television meteorologists predicting one to three inches of snow told viewers to stock up on supplies.

The last significant snowfall in Houston was in December 2009 when a half-inch covered the city.

Whatever snow falls, however, will melt quickly since temperatures will quickly rise into 60s, giving children precious little time to enjoy what might or might not be a winter wonderland.

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