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California Grid Operator Preparing for October Solar Eclipse

The California Independent System Operator has taken a number of steps to prepare for the effects of a solar eclipse that will occur next month.

CAISO recently released a technical bulletin related to next month’s solar eclipse, noting that on Saturday, October 14, 2023, an annular solar eclipse will pass over the western United States including California, Oregon, Nevada, Utah, Arizona, Colorado and New Mexico.

The bulletin details the expected impact of the solar eclipse, identifies the risks and possible measures to be taken by CAISO, the Western Energy Imbalance Market, Reliability Coordinator West and other balancing authority areas as well as market participants and stakeholders to address the loss of solar generation during the eclipse.

CAISO said that the WEIM, which covers much of the West, will be affected by the eclipse from 8 a.m. through 11 a.m. Pacific Daylight Time on October 14.

Each WEIM area will have varying times and magnitude of impact from the eclipse, with locations beginning between 8:05 a.m. and 8:15 a.m. and ending between 10:40 a.m. and 11:15 a.m. PDT and sun obscuration ranging from 65- 90%.

The California BAA will be affected by the eclipse from approximately 8:05 a.m. to 10:57 a.m. PDT. The range that the sun will be obscured varies from 89% in Northern California to 68% along the Southern California coast, according to the bulletin.

The reduction in solar radiation will directly affect the output of photovoltaic generating facilities, behind-the-meter rooftop solar, load, and net load within the CAISO BAA, as well as the WEIM.

U.S. Experienced Total Solar Eclipse in 2017

On August 21, 2017, the United States experienced a total solar eclipse that had impacts across the CAISO BAA and the Western Interconnection.

At that time, the CAISO had approximately 10,000 megawatts of installed capacity of commercially operational gridconnected PV solar and approximately 5,800 MW of BTM rooftop solar. Four utilities were participating in the CAISO WEIM services at the time of the 2017 eclipse.

“With proper planning, the CAISO was able to maintain grid reliability throughout the entire three hours of the eclipse,” the bulletin said.

Since 2017, grid-scale PV solar has grown to over 16,500 MW of installed capacity, and there is now 14,350 MW of BTM solar within the CAISO BAA.

Within the WEIM entities, grid-scale PV solar has grown from 866 MW in 2017 to 10,280 MW and BTM solar has increased from 738 MW to 6,458 MW.

Since then, the CAISO has also started offering reliability coordination services to the bulk of balancing areas in the Western Interconnection as the RC West, and there are now 21 balancing areas that participate in the WEIM.

“With the West more interconnected now than ever, the CAISO is evaluating the effects of the 2023 solar eclipse with a broader scope. Impacts will be felt across all of the utilities at various times and magnitudes within the WEIM and RC West, and entities can utilize these connections and relationships to maintain reliable operations, collaborate and optimize resources throughout the eclipse,” the bulletin said.

Growth in Solar Capacity

Assuming clear sky conditions, initial estimates show a number of impacts and planned mitigations for the CAISO BAA, the bulletin said.

The October 2023 eclipse will be more impactful than the 2017 eclipse because of the growth in solar capacity since 2017.

CAISO grid-scale and rooftop BTM solar have grown by 6,500 MW and 8,550 MW respectively since 2017. WEIM grid-scale and rooftop BTM solar have grown by 9,414 MW and 5,720 MW since 2017.

For the CAISO Balancing Authority on October 14, the eclipse will start to impact grid-scale solar and load at 8:05 a.m., reaching maximum impact at 9:30 a.m. before returning to normal conditions at 11:00 a.m.

“Precise timing and levels of obscuration will depend on geographic location relative to the eclipse path,” the bulletin points out.

The CAISO BA has approximately 16,500 MW of installed grid-scale solar capacity and 14,350 MW of rooftop BTM solar capacity. “Obscuration from the eclipse will significantly reduce generation from all solar resources. The change in grid-scale solar generation and gross load is largest on the eclipse return.”

Grid-scale solar generation will decrease by 9,374 MW from the start of the eclipse to the maximum impacted time period and increase by 10,801 MW on the eclipse return.

Gross load will increase by 2,374 MW from the start of the eclipse to the maximum impacted time period and drop by 6,643 MW from the eclipse maximum to the eclipse end.

Net load ramp rates on the eclipse return are larger than on the eclipse start to maximum impacted time period.

At the eclipse start to maximum impacted time period, the net load ramp rate will increase by an average of +122 MW per minute and decrease by -190 MW per minute on the return.

CAISO said WEIM regions will see varying eclipse impacts on renewables generation and load depending on levels of installed grid-scale and BTM rooftop solar.

The Central region has the largest amounts of installed grid-scale solar capacity, while the Desert Southwest region has the largest total installed BTM solar capacity.

The CAISO will coordinate with RC West, Utility Distribution Companies, and WEIM entities to ensure stable market operations to support reliable operations on the eclipse day.

CAISO and WEIM renewable scheduling coordinators should ensure appropriate eclipse impacts in their forecasts, the bulletin said.

Grid-scale solar resources will utilize specialized procedures to manage eclipse ramping requirements.

The specialized solar procedures include the potential to limit the solar generation ramp rate during the return of the eclipse, in addition to utilizing an operating instruction for the fleet to ensure solar resources are following their dispatch operating target and linearly ramping from Dispatch Operating Point to Dispatch Operating Point.

CAISO said that much of the success of the 2017 eclipse event was due to the accuracy with which solar generation was forecasted in the day-ahead timeframe, which limited the need for real-time redispatch.

“Due to the expected impact on solar resources, it is critical that solar resources within CAISO and the WEIM footprint account for the eclipse impacts in their market submittals,” the bulletin said.

CAISO “will ensure coordination with hydro and battery resources to safeguard availability to assist with large ramps expected on the system during the eclipse. Similar to the growth of solar since 2017, CAISO has had significant growth in battery resources to assist with the faster ramping needs on the system, and the CAISO plans to utilize battery and hydro resources during the solar eclipse.”

CAISO will procure additional operating reserve requirements to assist during the eclipse.

During the 2017 eclipse, additional reserve procurement helped grid operators handle resource deviation in addition to large ramps on the system. For 2023, the CAISO will procure additional operating reserve requirements to assist with the planned movement in solar generation, in addition to the potential for cloud cover to impact the system.

To get the most benefit while minimizing risk, greater coordination and preparation across utilities is required throughout the RC West and WEIM footprints as well as the CAISO balancing area, the bulletin noted.

While there have been many changes and additions within the CAISO BAA, WEIM and RC West since the 2017 eclipse, CAISO said it “remains committed to providing details on potential impacts and actions to allow entities to prepare for the eclipse, as well as support to ensure reliable operations throughout the event.”

Other Grid Operators

As for other grid operators, the Electric Reliability Council of Texas “expects the grid to operate under normal conditions and will continue to keep the public informed leading up to the event,” the grid operator said in a statement to Public Power Current.

While New York ISO forecasters “expect to see some reduction in solar generation during the October 2023 and April 2024 eclipse events, those reductions will not occur during peak demand,” said NYISO spokesperson Andrew Gregory.

We’re not expecting any major impacts in New England from the upcoming eclipse, given the path,” a spokesperson for ISO New England said. “We’ll of course be monitoring conditions, and gathering information on how the eclipse affects operations in other parts of the country ahead of the April 2024 eclipse, which will more directly impact this region.”

The Southwest Power Pool "is anticipating very minimal impact to our footprint" from the solar eclipse, said Meghan Sever, Senior Communication Strategist at SPP.

"This is due two main factors: We currently have a limited amount of solar resources (245 MW of solar capacity), and the direct path of the eclipse is expected to be south of our balancing authority footprint," she said.


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