About 25 percent of region's energy fleet was unavailable during polar vortex

The electricity grid in Minnesota and much of the Midwest was pushed to its limits during late January's frigid blast, as power plants suffered significant generation shortfalls due to extreme temperatures.

As the mercury plummeted to 20 below and colder, wind-turbine blades stopped spinning. Natural gas shortages surfaced. And both gas-fired and coal-fired power plants appeared to have some mechanical problems.

Indeed, about 25 percent of the region's electricity-generation fleet was unavailable because of forced outages, according to the Midcontinent Independent System Operator Inc. (MISO), the federally sanctioned nonprofit that operates the grid in parts of 15 states, including Minnesota.

But while generation capacity shrunk, MISO said it managed the situation so that widespread blackouts weren't a problem. "We kept the lights on," said Ron Arness, director of MISO's central region operations.

Arness gave a report on the deep freeze Wednesday at a meeting at MISO headquarters in Carmel, Ind. Representatives of MISO and eight Minnesota utilities are scheduled Thursday to appear at a hearing in St. Paul to discuss the cold snap between Jan. 28 and Feb 1.

The Minnesota Public Utilities Commission has asked the electricity and gas providers to identify and discuss weather-related service issues during the big freeze.

Minneapolis-based Xcel Energy experienced the most-publicized problems after about 150 of its natural gas customers near Princeton temporarily lost their heat. Xcel, the state's second-largest gas utility, also asked all of its 400,000-plus gas customers to conserve fuel by turning down their heat to 63 degrees.

Temperatures fell to 30 below or colder in Iowa and Illinois, 40 below in Wisconsin and "even worse" in Minnesota and the Dakotas, Arness said.

Electricity demand effectively rose, while electricity supply started blinking off. "We had a significant amount of wind, gas and coal generation forced out of service," Arness said.

Wind speeds were less than forecast on those days, meaning power production was less than expected. Ice on turbines hampered wind production. And then there was the brutally low temperature.

Wind turbines are often programmed to shut down when the thermometer is 20 below to 25 below because equipment can be damaged if turbines continue running. (The same is true during violent storms, when turbines are also programed to shut off at certain wind speeds.)

Xcel, Minnesota's largest electric utility, receives 21 percent of its generation from wind in Minnesota and the Dakotas, and its turbines automatically shut down around 20 below. They were generally down during the nights of Jan. 29 and 30, the company said in statement.

Meanwhile, some gas-fired power plants faced fuel shortages and mechanical problems due to the cold, Arness said. Coal plants took a hit, too. Arness speculated that frozen coal — the black stuff sits outside in big piles — and component failures cut generating capacity.

Xcel said it didn't have any outages at its coal or gas-fired plants due to cold weather.

MISO data showed that far more coal- and gas-fired generation was forced to shut down than wind during the deep freeze, though wind comprises a smaller percentage of generating capacity in the first place.

On Jan. 30, MISO declared a "Max Gen Event," something that has occurred eight times since 2013. In a Max Gen Event, MISO is faced with a generating-capacity emergency.

MISO partly allayed the situation by successfully importing a lot of electricity from other regional grids, or "ISOs," Arness said. Electricity producers outside MISO reacted to soaring wholesale prices and sold power into the MISO grid.

The grid operator asked them to fire up plants that normally would be used only in an emergency. These plants are generally "not available economically" — i.e. they are too costly to run — or they can run for only short periods due to environmental restrictions, Arness said.

"We didn't leave any generation on the table," he said. "By the end of the event, we had called on about everything."