Duke Energy officials told state regulators Tuesday that their computer models underestimated power demand by 6% to 10% during a Christmas Eve cold snap. Combined with a shortage of electricity from power plants, this contributed to the need for unexpected rolling blackouts across North Carolina at a time when temperatures dipped into the single digits.
The cold weather hit as Duke crews were still repairing damage from a winter storm that knocked out power across much of the country.
About 500,000 customers across the Carolinas were affected by the planned outages — some with little notice. Duke had said power would be out for 15 to 30 minutes. But officials acknowledged Tuesday that some customers were out for hours when its automated system failed.
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The outages started around 6:15 a.m. on Dec. 24. Workers began manually restoring customers at 8 a.m. and had all back online by 4 p.m. Christmas Eve.
"This was the first time in our history that we have to implement rolling service disruptions," said Kendal Bowman, Duke's state president for North Carolina.
Julie Janson, who oversees Duke's North Carolina and South Carolina operations, said "rapidly evolving events" made rolling outages necessary. Disconnecting some customers prevented damage to the grid, which would have caused more widespread and longer disruptions, she said at a meeting of the North Carolina Utilities Commission in Raleigh.
"We regret not being able to provide customers with as much advance notice of the outages as we would've liked. And we acknowledge that the outages themselves lasted far longer than we expected," she said.
Janson said Duke is conducting an internal review and studying how other utilities responded to come up with improvements.
"We own what happened. We have set out on a path to ensure that if we're faced with similar challenges, we will see a different outcome," Janson said.
Fast-moving breakdown
Duke officials provided a timeline of what happened leading up to Christmas Eve and the outages. It was a cascade of problems, including increased demand for power, less power available than expected, and malfunctioning power plants and computer systems.
On Friday, Dec. 23, Duke had about 1,500 megawatts of reserve power, which its forecasts showed should have been enough for early Saturday.
But temperatures early Saturday, Christmas Eve, were colder than either Duke or the National Weather Service predicted. That led to an increase in electricity demand for heating.
Meanwhile, Duke had 10% less power available than it had expected. That was because of malfunctions that reduced generating capacity at Duke's Dan River, Roxboro, Mayo and Lincoln County plants — all coal- and gas-powered.
"Between midnight (Friday) and early Saturday morning, we lost around 1,300 megawatts of power generating capability due to equipment malfunction," said Bowman.
At the same time, electricity that Duke had contracted to buy from out of state was not available, as other power companies faced similar demand. So Duke operators decided the only solution was to start selectively reducing electricity on the grid, using a system that stops and restarts power automatically.
But that automated system also failed. Duke's operators had to manually restore power, leaving many customers offline for much longer than expected, Duke officials said.
Utilities Commission chair Charlotte Mitchell said that while she appreciates that Duke Energy operators had to deal with rapidly changing conditions, "Your description of the events the night of the 23rd and then the wee hours of the 24th are concerning and scary."
"I also want to say we need to understand what happened here so that we can protect against this going forward," Mitchell said.
North Carolina regulators aren't the only ones asking questions. Federal officials have also announced an investigation into the cold-weather outages at Duke and other utilities.
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