March 11, 2014

Cold snap nearly triggered rotating outages

According to a report from the Electric Reliability Council of Texas, 97 power sources experienced issues Jan. 6 ranging from frozen instrumentation to fuel restrictions.

Nearly 100 electricity generators had problems producing power at some point during a Jan. 6 cold snap that pushed the state’s biggest power grid into an emergency alert and close to rotating outages.

The report from the Electric Reliability Council of Texas details its minute-by-minute performance that day during what it terms a Level 2 energy emergency alert. By the end of the day, 97 power sources on ERCOT’s system had experienced some sort of issue, tripped offline, failed to start or were “derated,” meaning their output was reduced.

ERCOT serves about 85 percent of Texas electricity demand, including most of North Texas. According to its report, released Friday:

• Generator outages specifically tied to the weather hit units representing 3,541 megawatts of capacity. Most of that, units with close to 3,000 megawatts of capacity, was due to “frozen instrumentation.” (A unit is an individual machine generating electricity; there can be several units at a large power plant.)
• An additional 1,643 megawatts, all at natural-gas-fired units, were unavailable because of restrictions in their fuel supply.
• Output at more than 30 wind farms was derated by more than 700 megawatts total because their turbines reached their “low temperature limit.”
• Still more generators were unavailable because of other equipment failures or scheduled maintenance.

A megawatt is enough electricity to serve about 200 Texas homes during a period of peak demand, typically a blazing summer day.

Temperatures were generally in the 20s on Jan. 6, and the peak demand was just short of what the state saw on Feb. 2, 2011, the last time ERCOT experienced rotating outages. It has declared emergency alerts and energy conservation appeals several times since.

Michael Goggin, senior electric industry analyst with the American Wind Energy Association, said the low temperature limit on turbines is related to the performance of lubricants and concerns about the brittleness of components on Texas units, which don’t have the same cold-hardened designs used in colder climates.

Goggin noted that “wind was only 5.7 percent of the capacity taken offline by the cold, even though wind is about 15 percent of the state’s capacity, so wind energy fared far better than the average for other fuel types.”

On Tuesday, the Sierra Club said the experience “shows that clean-energy solutions, especially clean, cheap Texas wind and demand response performed as expected and under pressure, whereas numerous coal-fired and gas-fired power plants across the state couldn’t.”

ERCOT enters a Level 1 energy emergency alert when it has less than 2,300 megawatts of capacity in reserve, or generation supply available if needed. It goes to Level 2 when that reserve drops to 1,750 megawatts, prompting it to tap idle generation and cut off big commercial users that have agreed to have their power curtailed at such times. Level 3 is rotating blackouts, where local utilities drop customer load piecemeal to avoid a wider blackout.

ERCOT said its available capacity slid from more than 61,000 megawatts the afternoon of Jan. 5 to just over 56,000 at 7 a.m. Jan. 6. Its reserve bottomed out at 1,344 megawatts at 7:09 a.m., according to the report.

At the same time, ERCOT said that the previous evening, it had underestimated demand going into Jan. 6 by about 2,400 megawatts. It said that by 4 a.m. the morning of Jan. 6, its expected load “was closer to the actual load for most of the day” because of updated temperature forecasts.

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