Renewable power with smart inverters compete with traditional power plants

The California ISO said that testing conducted last August on a 300 MW First Solar photovoltaic plant demonstrated that renewable energy plants with smart inverter technology can offer electric reliability services similar, or in some cases superior to, conventional power plants.

According to the California ISO’s statement, the ISO, the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL), and First Solar analyzed the plant for performance in three critical areas: frequency control; voltage control; and ramping capacity.

The California ISO said that NERC has identified those services as essential for expanded integration of renewable resources into the power grid.

According to the report, “Using renewables to operate a low-carbon grid: Demonstration of advanced reliability services from a utility-scale solar PV plant,” the tests conducted various forms of active power controls, including automatic generation control (AGC) and frequency regulation, droop response, and reactive power/voltage/power factor controls.

The report noted that the purpose of the AGC tests is to enable the power plant to follow the active power set points sent by the California ISO’s AGC system; the set point signal is received by the remote terminal unit in the plant substation and then scaled and routed to the power plant controller (PPC) in the same time frame.

The AGC tests demonstrated the plant’s ability to follow the California ISO’s AGC dispatch signals during three different solar resource intensity timeframes: sunrise, middle of the day, and sunset, the report said. For that purpose, the plant was curtailed by 30 MW from its available peak power to have maneuverability to follow the California ISO’s AGC signal, the report said. During those tests, a fast and accurate AGC performance has been demonstrated at different solar resource conditions, the report said.

For frequency response tests, the plant was also operating in curtailed model to have enough headroom for increasing its output in response to frequency decline outside of a defined dead band, the report said. Headroom is achieved by sending a curtailment command to plant PPC after initially computing its estimation of maximum capability using real-time solar irradiance data from, for instance, real-time measurements of panel and inverter data, the report said.

Assuming that the plant will be reimbursed for the energy loss due to curtailment for those ancillary services, the maximum power estimation will likely need to be refined and validated, the report said, adding that the plant demonstrated fast and accurate frequency response performance for different droop settings — 3 percent and 5 percent — under various solar resource conditions for under- and over-frequency events.

Among other things, the report noted that the plant also demonstrated the ability to operate in three reactive power control-related modes: voltage regulation, power factor regulation, and reactive power control modes. The plant can operate in only one of the three modes at a time with seamless transition from one mode to another, the report said, adding that the plant controller was able to maintain the specified voltage set points at the point of interconnection by regulating the reactive power produced or absorbed by PV inverters.

The test findings bring new tools to the challenge of ensuring system reliability while operating systems at a high level of renewable penetration, the California ISO said in its statement, adding that while the smart inverter technology is widely available, First Solar designed an advanced plant-level controller that is not commonly used in the industry now.

The California ISO noted that perhaps the most unexpected and significant benefit is the agile voltage support offered by the solar plant when it is generating during the day, and at night when it is not generating power; at night, the plant can absorb a small amount of power from the grid to provide reactive capability needed to support grid voltage.

The California ISO board of governors, which last month noted the findings as groundbreaking for advancing renewable integration in California, directed staff to develop market mechanisms to take commercial advantage of advanced inverter technology with the specialized controllers for reliable grid operation.

The California ISO added that it plans similar testing on a large wind plant, noting that since wind farms use similar smart power-based technology, it is anticipated that they will also be able to provide these types of grid reliability services.

Drawing reliability services from solar and wind power plants helps balance the grid during times of high renewable production and allows more clean energy to move onto the system, which is essential for California to be able to reach its goal of getting half of its energy from renewable sources by 2030, the California ISO said.

According to the report, the California ISO has more than 9,000 MW of transmission-connected solar resources within its operational footprint, and to meet its 33 percent renewable portfolio standard (RPS) by 2020, the California ISO is expecting an additional 4,000 MW to 5,000 MW of solar. Beyond 2020, to meet a 50 percent RPS, the California ISO is expecting an additional 15,000 MW of renewable resources, and a significant portion of that is anticipated to be transmission-connected solar PV due to the expected reduction in price of solar panels, the report said.

The California ISO said in its statement that it plans to present its test results to NERC and other technical review committees. The California ISO said that it will evaluate the grid’s solar fleet to determine the amount of existing capacity capable of providing essential reliability services, and that it will explore additional opportunities for those variable energy resources to join the California ISO’s power and ancillary services markets.

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Corina Rivera-Linares, chief analyst for TransmissionHub, has covered the U.S. power industry for the past 10 years. Before joining TransmissionHub, Corina covered renewable energy and environmental issues, as well as transmission, generation, regulation, legislation and ISO/RTO matters at SNL Financial. She has also covered such topics as health, politics and education for weekly newspapers and national magazines. She can be reached at .

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