Texas’s largest power producers are blaming last week’s blackouts on widespread failures of the state’s energy system.

Calpine Corp., Vistra Corp. and NRG Energy Inc. told Texas lawmakers Thursday that grid issues, natural gas-supply shortages and regulatory missteps all contributed to the power-plant outages that left more than 4 million homes and business without heat, light and water during a deep winter freeze.

“The entire energy sector failed Texas,” NRG President Mauricio Gutierrez said in opening remarks.

The historic outage caused as much as $129 billion in economic losses, and the impact to individual companies is only starting to emerge. Some electricity providers wracked up huge losses, fueling a possible credit crisis. Oil and gas producers saw their output halted. And dozens of people died.

Texas lawmakers holding simultaneous hearings on the disaster appeared frustrated over the clashing explanations of the disaster as they grilled the executives and the grid operator about what went wrong.

“Who turned my power off!” yelled Representative Todd Hunter, an attorney from Corpus Christi. “You guys are getting the heat because you’re the front burner!”

Frequency Issues

Calpine, Vistra and NRG all said they had plants forced offline after the flow of electricity on the grid—called frequency—plunged during the early morning of Feb. 15, when blackouts were first ordered. Their comments contradict the version of events presented by the Electric Reliability Council of Texas, which manages most of the state’s grid.

The grid operator, known as Ercot, has maintained that the dip in frequency was not significant enough to cause plants to trip.

“We have examined this, we haven’t seen it,” Chief Executive Officer Bill Magness said in testimony before the Texas senate. If plants did go offline in tandem with the dip, it would only have been around 10 units, a number dwarfed by the total that was offline due to weather and gas-supply issues, he said.

Maintaining frequency at around 60 hertz is critical to keeping the grid stable. Ercot operating protocols say a deviation of 0.2 hertz “for a long period” could cause damage to generators and customer equipment. On the day of the blackouts, frequency dipped to 59.4 hertz for 4 minutes and 23 seconds, according to an Ercot presentation. It fell as low as 59.3, according to Bloomberg data.

NRG’s Gutierrez said the dip “threatened the majority of the fleet” but ultimately only caused one plant to go offline. Calpine Chief Executive Officer Thad Hill said in written testimony that two of the company’s natural gas-fired power plants tripped offline for the same reason.

Vistra was within three minutes of losing Comanche Peak nuclear plant because of low frequency, Vistra Chief Executive Officer Curt Morgan said. “We came dangerously close to losing the system,” he said.

Gas Shortages

Vistra and Calpine also said gas supply shortages affected their ability to operate. While freezing weather shut in some gas production, the blackouts ordered by Ercot compounded the issue as power was cut from pipeline compressors necessary to transport the fuel to power plants.

“We just couldn’t get the gas” despite having 90% of plants available to run, Vistra’s Morgan said. “For us, it was not a winterization issue—it was a gas availability issue.” He also cited frozen coal and rail delays.

In addition, the state’s critical infrastructure plan wasn’t updated after the freeze of 2011, he said.