Grid Operators Manage Swings in Solar Generation During the Eclipse

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Grid Operators Manage Solar Eclipse

RTO Insider, August 21, 2017

CAISO and other electric grid operators across the country managed large and rapid swings in solar generation output Monday during the first continent-wide total solar eclipse in nearly a century.

ISOs and RTOs were well prepared for the event, especially in solar-heavy California where the obscuration of the sun took thousands of megawatts of utility and rooftop solar off the grid. CAISO had to ramp up hydro and natural gas generation as solar dropped off, then do the reverse more quickly than usual as the sun came back.

Electronic board in CAISO control room displays solar generation (left) and load (right) during and after eclipse | CAISO

“We wanted to make sure we could make it if it was an extremely hot day, or if it was a mild day,” CAISO Executive Director of Operations Nancy Traweek said. She added that the ISO had reached out to solar and hydro operators and asked them to be prepared for the event.

The last total solar eclipse to occur in the continental U.S. was before the growth in solar power in 1979 and was viewable only from the Pacific Northwest, according to NASA. Monday’s was the first total eclipse since 1918 to span the width of the U.S.

As eyes equipped with protective glasses turned upward around the country, CAISO employees excitedly gathered outside the building, some with family members, to view the event.

CAISO said it would not be able to provide precise figures for how much solar generation dropped off its system until later this week.

“We forecasted 4,200 MW of utility-scale solar coming off. We believe that the actual will be more in the 3,000 to 3,500 MW range,” CAISO spokesman Steven Greenlee said.

CAISO data showed that the eclipse took a little more than 3,000 MW offline; in a briefing Monday morning, ISO officials said more than 3,000 MW of utility solar and 1,400 MW of rooftop solar could be lost.

Grid operators had to deal with two solar ramp-ups rather than just one.

About 10:50 a.m. PT, after totality, load was about 30,500 MW and solar generation was about 4,100 MW, with the grid stable. When the sun was nearly clear of the moon about 11:30, CAISO said load was about 29,300 MW and solar generation was about 6,800 MW. By about 1:30 p.m., solar generation in the ISO was back up to about 9,000 MW. There is about 10,000 MW of solar capacity on the ISO system.

CAISO had to manage not only the rapid loss of solar but also a steeper-than-usual climb of that resource compared with a normal day as the sun returned. CAISO predicted it would lose about 51 MW/minute, and as the blockage waned, solar generation came back at a rate of 93 to 100 MW/minute. On a normal morning, solar ramps about 29 MW/minute.

Wholesale prices briefly went negative as solar returned, as they normally do when there is excess generation on the grid. CAISO said that the 1,000-mile East-West span of the Western Energy Imbalance Market (EIM) allowed it to call on available resources as other areas ramped down.

About 860 MW of solar went off the grid in the EIM.

SPP, ERCOT See Little Impact

SPP had anticipated a peak load of approximately 45,000 MW across its system Monday but saw demand about 2,500 MW below that as air conditioning usage dropped and manufacturing facilities closed while employees observed the eclipse.

“In preparation for the relatively sudden and not entirely predictable drop in load, SPP utilized its day-ahead market processes beginning Aug. 20 to commit adequate reserves to accommodate load swings and the resulting impacts to frequency and interchange,” SPP said. The RTO increased its regulation service in preparation. An eclipse also slows wind speed by cooling air, causing a 1,200-MW swing in the RTO’s wind generation that also had to be managed.

“By increasing our regulation requirements, we essentially ‘widened the lanes’ of our system and operated more conservatively than we might have on a normal day to accommodate any unpredictable occurrences during this rare event,” Director of System Operations CJ Brown said.

This was a great learning opportunity for SPP,” said Vice President of Operations Bruce Rew. “And I’m proud that our staff and systems were able to ensure that, despite so many variables and the rarity of the solar eclipse, it was essentially a non-event electrically speaking.”

Utility-scale solar in the ERCOT system dropped from a peak of 760 MW to a low of 299 MW during the eclipse, while total system load dropped from 60,824 MW to 60,163 MW. The ISO said a number of factors could have contributed to the load decrease, including reduced air-conditioning demand.

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