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Curtailment Tracker: Cal-ISO wind, solar curtailments in June drop on more load

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Curtailment Tracker: Cal-ISO wind, solar curtailments in June drop on more load


Curtailments dropped 72% month on month

Hydro generation output down 39% year on year

Loads 2 GW higher during solar curtailment hours

California Independent System Operator wind and solar generation curtailments dropped 55% year on year in June on increased systemwide demand, lower imports and less hydro generation, according to grid operator data.

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Localized curtailments accounted for about 88% of the renewable generation curtailed in June and made up 78.2% of curtailments from January through June, according to ISO data. In comparison, localized curtailments accounted for 67% of the 1,254,326 GWh curtailed in June 2020, according to ISO data.

There are two types of curtailments. Systemwide curtailments occur when there is an oversupply, and localized curtailments occur when the amount of power from one transmission point to another point is controlled for congestion management.

ISO curtailments plunged 72% month on month to 92.375 GWh in June as ISO peak demand reached 41.397 MW June 17, an increase of 29% month on month, according to the ISO's June Key Statistics report. Curtailments reached an all-time record of 341.959 GWh curtailments in March.

"During the hours when solar curtailments are most likely to occur - late morning to early afternoon - loads were about 2,000 MW higher this year, imports were 1,700 MW lower and hydro generation was about 800 MW lower," Morris Greenberg, senior manager of North American Power for S&P Global Platts Analytics, said about the year-on-year comparison. "In addition, we have been seeing several hundred MW of battery charging."

Combined, this would allow the ISO to absorb nearly 5 GW of incremental solar generation on average, which exceeds incremental potential supply, Greenberg added. Since June 2020, 1.043 GW of battery storage has been added to the Cal-ISO grid.

Generation changes

Systemwide peakload was up 5.3% year on year to average 34.488 GW in June, according to ISO data. Imported generation decreased 17% year on year to average 126.916 GWh/d in June, as hydro generation output sank 39% year on year to average 35.749 GWh/d.

Solar generation output increased 11.5% to average 123.523 GWh/d in June as wind generation output rose 7.7% to 69.843 GWh/d, according to ISO data. Meanwhile, thermal generation jumped 8.6 points year on year despite SoCal city-gate spot gas prices climbing 157% year on year.

Cal-ISO installed renewable capacity increased 9% year on year in June to 25.558 GW with solar accounting for 57.2% and wind making up 28.8%, according to the Key Statistics report. The remaining 13.9%, or 3.456546 GW comes from geothermal, small hydro and biofuels.


So far this month, wind and solar curtailments have totaled 20.170 GWh through July 18, 9.5% higher than the same period in July 2020, according to ISO data.

SP15 on-peak balance-of-the-month was bid at $75/MWh on the Intercontinental Exchange July 19, nearly 17% lower than where the daily traded. The SP15 on-peak July package averaged $130.79/MWh in its last month, 247% higher than the 2020 package average a year ago due to summer supply concerns with a weak hydro outlook, continued drought and above-normal temperatures, according to S&P Global Platts data. On-peak August is currently in the mid-$130s/MWh with September in the mid-$100s/MWh, 250% and 188% higher than 2020 packages a year ago.

There is a greater probability for above-normal temperatures across California in the three-month temperature forecast, according to the US National Weather Service.