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PJM, CAISO detail load changes during solar eclipse

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The California ISO lost about 3,500 MW of grid-connected solar generating capacity and the PJM Interconnection lost 520 MW during last month's solar eclipse, the two grid operators said in separate reports Sept. 12. About 23,000 MW of utility-scale solar was in the path of the eclipse.

CAISO also estimated that 1,300 MW to 1,500 MW of distributed solar projects, such as rooftop systems, went offline during the eclipse, which out west occurred during the morning hours Aug. 21. PJM estimated about 1,700 MW of distributed or "behind-the-meter" capacity went out of service during the eastern region's midafternoon hours.

PJM was prepared to lose up to 2,500 MW of solar output but lost less distributed capacity, Joe Mulhern of the grid operator's generation department said in a presentation at a PJM Operating Committee meeting Sept. 12. Overall power consumption dropped by about 5,000 MW in PJM during the eclipse, in part due to temperatures dropping by about 3 degrees Fahrenheit during the roughly two-hour event, he said. Committee members also expected that lower air conditioning demand contributed as well.

Ken Seiler, PJM's executive director for system operations, said during the committee meeting that he plans to meet with "smart" thermostat operator Nest Labs to get their take on load changes. Nest Labs had a "Solar Eclipse Rush Hour" program during the eclipse, where customers who have the thermostats could opt to use less energy. Nest Labs, in an Aug. 10 blog post that was updated after the eclipse, said the program helped reduce about 700 MW of power demand nationwide.

California

CAISO has about 16,000 MW of solar capacity installed on its system, including rooftop sources, it said in a Sept. 12 news release. As the eclipse began at 9:11 a.m. PT, solar production was at 6,392 MW, but dropped by more than half in a little more than an hour, declining at a rate of about 48 MW per minute. After the peak obscuration, solar resources returned at a rate of 150 MW per minute, CAISO said. By 11:17 a.m. PT, the grid operator had to begin curtailing solar production, which it said is common on high-solar days.

The grid operator noted that wholesale market price fluctuations as well. "The average wholesale electricity cost was $16.79 per MWh at the beginning of the eclipse, rose to $22.27 at maximum obscuration, then normalized to $16.15 by 11:17 a.m." local time, CAISO said in its release. The uptick in price was expected because higher-priced, "fast-ramping resources were needed to fill demand and stabilize the system," CAISO said.

Unlike in PJM, demand in the CAISO region increased during the eclipse. The grid operator said the demand pattern was normal for that time of day and expected.

During the eclipse, the California grid operator imported about 3,000 MW of capacity. Of that amount, about 650 MW came from the energy imbalance market, a real-time trading market in the western U.S. About 1,600 MW came from thermal generation and 791 MW from hydro, CAISO said in the release.

Legal perspective

Steven Ross, a partner at Steptoe and Johnson LLP, who has decades of experience representing investor-owned utilities and electric power companies, said careful planning helped avoid contractual issues with commercial solar projects during the eclipse.

For power producers who had purchase obligations for commercial solar, "they often were able to curtail those resources ahead of the eclipse so they could ... have the outages more orderly staged ... [and] have their replacement resources ready to go and ramp up in parallel as the solar resources were being ramped down," Ross said in an Aug. 23 interview, clarifying that he was not speaking on behalf of any specific client.

When asked how common it is for power contracts to account for rare events like an eclipse, Ross said no contract is one-size-fits-all. It is common for third-party contracts to build in provisions that address interruptions and curtailments, which generally give a utility or purchaser the right to not take power in certain instances when grid operators determine substantial potential for adverse impacts on system reliability, Ross said.

Utilities that own a solar project and the transmission system the project is connected into will "have maybe a contract provision in their purchase contract that discusses what I just described," Ross said. "If it is a situation where the solar person is wheeling through to someone else over the transmission service provider's system, there may also be a provision in the interconnection agreement between the solar provider and the transmission wires owner that says, I, too, independently of your purchase contract, I, too, can open our ties under circumstances where I detect a potential system reliability or safety problems."