As bitter cold moved through, Xcel's natural gas system pushed its limits

Xcel Energy Co.'s natural gas distribution system revealed its limits this week when Arctic cold blew through the state, leading to outages in about 150 homes and a request from the utility that all customers lower thermostats for Wednesday night.

Xcel, the state's largest supplier of electricity and second-largest provider of natural gas, is already planning changes to its natural gas network, which supplies more than 400,000 households. In Princeton, Minn., where an outage to dozens of homes lasted for about a day, Xcel will likely begin construction soon to add more capacity to affected neighborhoods.

"We'll be adding another pipe from a different direction to supply more gas into the area," Kent Larson, Xcel's executive vice president of operations, said Friday.

Xcel designs its natural gas system to deliver all the fuel customers want 100 percent of the time. But that design is based on estimates that are only really tested when demand spikes, as it did during this week's polar vortex, which dropped low temperatures into the -20s and -30s for much of the state Tuesday through Thursday.

"You do all these calculations ahead of time," Larson said. "But when you have the real situation happen, you'll learn some things and make some improvements."

As temperatures moderated Thursday, demand for natural gas eased, heat was restored at the homes in Princeton and the utility lifted its request that customers lower thermostats.

The designers of gas transmission systems face the same challenge as highway planners; peak capacity is rarely reached and, when it is, the strain creates limitations that disrupt and upset customers.

Gov. Tim Walz; the Minnesota Office of Pipeline Safety, an arm of the state's public safety department; and the state Public Utilities Commission, which regulates utilities; have all asked Xcel for more details about Wednesday's problem.

Rep. Jean Wagenius, DFL-Minneapolis, chairwoman of the energy policy division of the House Ways and Means Committee, said Friday that she had been following the news about the Xcel constraints. "The information being gathered by the PUC and others is going to help us decide whether we need to do a hearing," she said.

Natural gas fuels boilers, furnaces and water heaters in homes and businesses. It is also used by Xcel and other utilities to run power-generating stations that provide electricity. The fuel is abundantly produced in the U.S., and Xcel had no difficulty obtaining it to deliver to customers this week.

Instead, the difficulty emerged in Xcel's distribution system, which reached its full capacity on Wednesday afternoon when many people were at home with furnaces that were running nearly constantly.

"On a zero or -10 day, they're running half or two-thirds of the time," Larson said. "With everyone running at 100 percent, the draw on the system is substantially more than it would be otherwise."

In the afternoon, engineers saw the broader system was at full capacity and asked customers to lower thermostats to 63 degrees for the night.

"Because we were right at the limit of the system, and maybe even slightly beyond, we made the decision with an abundance of caution to ask customers to cut back," Larson said. "We hadn't seen these temperatures in two decades and decided to be a little cautious that way."

The risk was that the surging demand for fuel would lower pressure at the endpoints of the network, as happened in the Princeton homes. The amount of gas that can flow through a pipe is always the same; only the pressure changes when more is wanted.

At its most dangerous, low pressure in the gas line can cause pilot lights to go out, allowing gas to flow out into homes and businesses where it becomes a combustion risk.