HOUSTON, Jan 8 (Reuters) - PJM Interconnection, the power grid agency for 13 mid-Atlantic and Midwestern states, said power plants knocked offline earlier this week by frigid temperatures are slowly returning to service as the region begins to thaw.
PJM said it expected 27,000 megawatts of generation would be offline Wednesday evening, an improvement from the 39,500 MW that were shut earlier in the day.
The amount of natural gas-fired generation expected to be offline due to curtailments of gas - about 9,000 MW - showed no improvement from earlier in the day, PJM said in an email detailing this week's power-plant problems.
Wednesday morning's outage figure represents nearly 21 percent of PJM's total generating capacity of 189,658 MW. In late December, before the sub-freezing temperatures moved into the central U.S., PJM outages were running at about 12 percent of total generation, according to grid data.
PJM took emergency action Tuesday to meet surging demand as subzero temperatures in the region forced many power plants offline.
Overall, PJM said between 35,000 and 38,000 MW were offline Tuesday, up from 30,200 Monday evening. Actual outages fluctuate throughout the day as operators shut plants, make repairs and return them to service, officials said.
Michael Kormos, PJM's executive vice president for operations, cited weather-related mechanical failures and natural gas supply problems, as well as normal generation issues, for the increased power-plant outages during the bitterly cold time period.
"We've seen everything," Kormos said.
In its report, PJM said 19,114 MW of steam boiler plants, most coal-burning units, were shut early Wednesday when overall outages were above 39,500 MW.
More than 16,000 MW of natural gas and diesel-fired plants were unable to operate, including nearly 2,300 MW of newer, combined-cycle, gas-fired plants.
About 9,000 MW of gas generation was affected by gas pipeline curtailments, PJM said, describing the amount as "a relatively small percentage of the total outages."
Another 1,600 MW of nuclear generation was shut and nearly 1,500 MW of wind.
Despite emergency steps taken to curb electric use Tuesday, PJM set two winter power use records that day, of 138,000 MW in the morning and 141,312 MW later that evening, PJM said in a release. Power use exceeded PJM's previous record of 136,675 MW set in 2007 by 3.3 percent.
"Our thanks go to all of the consumers who heeded our call yesterday to conserve electricity," said Terry Boston, PJM president.
Wednesday's PJM peak demand eased to 134,505 MW and Thursday's peak will fall further to 122,341 MW, according to the PJM website.
Next-day power prices in PJM on Wednesday retreated to the mid-$50s per megawatt-hour - the five-year average for January - from an average of $240 per MWh the previous day.
Companies that generate power in PJM include units of American Electric Power Co, NRG Energy, FirstEnergy Corp, Exelon Corp, Public Service Electric & Gas Co and others.