Xcel Energy last week experienced a major gas pipeline problem in Colorado, nine days after the company had to cut natural gas to about 150 customers in Minnesota due to low pressure in its distribution system here.

Low pressure in one of Xcel's Colorado gas transmission lines Friday prompted the company to institute short-term electricity brownouts that affected 17,600 customers in the Rocky Mountains. Xcel initially suspected a breach in the gas transmission line.

"When we see pressure dropping it's an indicator — is there some kind of breach in the pipeline where gas is coming out?" said Jerome Davis, regional vice president for Xcel in Colorado.

But after an investigation, Xcel this week ruled out a pipeline rupture in Colorado. The company is still investigating the cause of the pressure drop, focusing on its own operating procedures.

Minneapolis-based Xcel is both an electricity and gas provider. Colorado and Minnesota are its largest markets.

During the last week of January, bitterly cold weather increased heating demand in Minnesota, straining Xcel's gas distribution system to the point where the company cut service to about 150 customers in the Princeton area.

Xcel also asked its nearly 870,000 Minnesota gas customers to temporarily lower their thermostats to 63 degrees to conserve fuel.

Events such as those in Colorado and Minnesota are rare, and the causes appear to be different.

On Friday morning, Xcel discovered low pressure in its natural gas system serving Breckenridge and four other communities in Summit County, Colo.

In response, Xcel said it undertook controlled, rolling outages for periods of 30 minutes, generally affecting several thousand customers at a time, but not more than 17,600 altogether. Summit County schools were closed, and several ski resorts were affected.

Xcel said the electricity outages were aimed at conserving natural gas in the Breckenridge area. When the power was out, customers temporarily could not turn up their furnaces since electricity is needed to do so.

"People aren't using as much natural gas as they would if their electricity was on," Davis said. "What the [controlled outages] did was give us time. It kept the pressures up."

No Colorado customers lost gas service, he said. If they had, Xcel would have had to go from home to home relighting pilot lights, as it did in Princeton.

The gas lines serving Princeton were distribution lines. In Colorado, the pressure drop occurred in a transmission pipeline, a big high-pressure line that feeds distribution systems.

A breach on a transmission line would cause an all-out pipeline rupture, not just a gas leak, said Richard Kuprewicz, president of Accufacts, a Seattle-based pipeline safety consulting firm. "You have to really suck a lot of gas out to have the pressure drop."

Ruptures can spawn explosive fireballs. A breach also can fail to ignite, yet still cause a destructive blast, Kuprewicz said.

Xcel commissioned a helicopter to fly over the affected transmission line in Colorado, looking for a rupture.

Surveyors used lidar, a laser light technology that can detect natural gas pipeline leaks. Inspections found no breach.

The company also has ruled out curtailment issues with its natural gas suppliers, another possible cause of a pipeline pressure drop.

So now the company is looking at its internal procedures, Davis said.

Xcel must make a report of the incident to the Colorado Public Utilities Commission, which said in a statement it would determine if any "appropriate follow-up steps" are needed. Xcel also must file a report regarding the Princeton outages with the Minnesota Public Utilities Commission.

The Minnesota Office of Pipeline Safety also is investigating the Princeton outage.