Rolling blackouts are generally considered a scourge of hot summer days when electricity demand for air conditioning pushes generation and transmission beyond capacity. However, electric utilities that rely on gas-fired generators are vulnerable during the winter due to the potential for gas well freeze-offs. Xcel Energy and some of its Colorado customers found this out over the holiday weekend.

On Saturday, Feb. 18, Xcel, which is headquartered in Minneapolis, reported curtailed gas supplies along Colorado’s Front Range and asked customers to cut usage of both gas and power. Further, Xcel said it would curtail power to customer groups in the Denver metro area on a 30-minute rotating basis. While Xcel said this was expected to last all day Saturday, things didn’t turn out that bad.

Press reports said outages only lasted about an hour and a half as Xcel’s gas suppliers were able to bring production back up at their wells. Outages affected about 300,000 electric customers in the Denver area as well as Eagle, Grand Junction, Vail and Aspen, CO. A company spokesman said he thought this was the first time Xcel was forced to cut power during the winter months.

Last week Xcel spokesman Tom Henley said temperatures were 15 degrees colder than normal, 13-below zero in some areas. He also said that the outages were due to a combination of things. Besides freezing temperatures and well shut-ins, he said Xcel experienced some equipment failures on the power generation side. Henley said he did not know how many megawatts were affected or when repairs might be completed. “We’re not going to get into that,” he said.

The company lost 510 MW of coal-fired generation due to equipment failures. “And we lost a tremendous amount of natural gas-fired generation,” Henley said. “We lost quite a few thousand megawatts, not only on our own system but on the IPP [independent power producer] side as well.”

Because some pipelines serving gas-fired generation lost pressure, some of the company’s more efficient generating units wouldn’t run. Xcel had to switch to less efficient generators and, hence, burned more gas during a period of constrained supply than it otherwise would have needed. The utility was forced to buy both gas and power on the spot market, but Henley said he did not know how much was purchased.

“These are all things that we’re still in the process of compiling because we’ll have to work with the [Colorado] Public Utilities Commission to explain what happened.”

Henley said he was unaware of any problems afflicting electric transmission or distribution lines. Xcel’s last controlled outage was last summer in portions of Denver. It was caused by transformer failure at a substation.

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