In a new report on a year-long demonstration and testing of two utility-scale solar photovoltaic (PV) projects’ ability to address intermittency issues, the National Renewable Energy Laboratory (NREL) said utilities, grid operators and regulators should encourage solar project owners to have advanced inverters or features that control a project’s output to match grid reliability needs.
Having ancillary services or other market measures that ensure large solar PV facilities are rewarded for providing grid-friendly services will be key given the projections for growth in the wind and solar power sectors, NREL said in the Jan. 29 report.
The demonstration showed how active power controls can leverage PV’s value from being simply a variable energy resource to providing ancillary services that range from spinning reserves, load following, ramping, frequency response, variability smoothing and frequency regulation, NREL said.
The report is based on the operations of AES Corporation’s (NYSE:AES) Ilumina PV project, a 20 MW facility near Guayama in Puerto Rico, and First Solar Electric’s Pecos Barilla PV project, a 22 MW facility near Fort Stockton in Pecos County in West Texas.
The intermittency of renewable resources has gained attention from NERC and others as posing reliability concerns, with NERC issuing a report in December 2015 asserting that new generation resources must provide adequate levels of frequency support, ramping capability and voltage control to maintain reliability of the bulk electric system.
As solar PV penetrations increase, it will become more important that the projects do not harm grid reliability, NREL said in the report.
“Either mandating grid services or providing revenue for them via ancillary service payments is the next step to encouraging PV power plant owners to procure advanced inverter or power plant controller features,” the report said.
The ancillary services markets among independent system operators (ISOs) varies in what is required from resources, NREL noted.
“Regulators and market operators should ensure that plant owners are operating on a level-playing field with clear requirements or incentive streams,” and if “PV-generated power can offer a supportive product that benefits the power system and is economic for PV power plant owners and customers, this functionality should be recognized and encouraged,” NREL said.
Plenty of controlling features such as advanced inverters are available for PV project developers, but the power market design in the United States does not make it a simple decision for project owners to include them, NREL asserted.
“For example, a manufacturer of PV inverters may know that a particular capability is technically feasible, but they may not see a market for it because there is no demand from a developer or requirement from a utility off-taker for the provision of that capability,” the report said.
Similarly, PV project owners may know what features PV inverters are capable of, but they may not choose to procure them or offer them to an off-taker if that capability is not required or if it does not result in increased revenue.
In addition, ISOs may desire a particular capability to address intermittency, but they may be unsure about how a project will perform and if it will actually improve system reliability, NREL said.
Finally, regulators may not establish complementary policies or market designs if the markets are receiving enough controllable capabilities for free and they may not be considering how this may change in the future with a different resource mix, NREL said.
The demonstration project’s goal was to help close those gaps in perspectives by providing test data from the utility-scale PV facilities, and the facilities were chosen based on several criteria, NREL noted. That criteria included facilities of a moderate size and without energy storage, with existing active power and voltage controls so that the test results are applicable to a large variety of PV plants and power systems.
It also helped that the Puerto Rico Electric Power Authority and the Electric Reliability Council of Texas (ERCOT) expressed strong interest in the project because of the high penetration levels of variable generation in their markets, NREL added.
The high level of wind power being added in the Panhandle region of Texas is creating transmission system challenges such as wind power ramping rates causing voltage stability issues and sub-synchronous oscillation on the lengthy transmission lines that move wind power from West Texas to load centers farther east. The competitive renewable energy zones in Texas are being stressed, which ERCOT may have to address through additional infrastructure and expense, NREL said.
“From this perspective, the reactive power and voltage control capabilities of modern utility-scale PV inverters can play an important role in addressing and/or mitigating some of the voltage stability issues present in West Texas areas,” the report said.
At the First Solar project in Puerto Rico, a key component of the grid-friendly plant is the plant-level controller developed by First Solar, NREL said.
“Although individual inverters comprise the plant, with each inverter performing its own energy production based on local solar array conditions, the function of the plant controller is to coordinate the power output to provide typical large power-plant features,” such as voltage regulation and reactive power control, NREL said.
As the proportion of solar PV in the generation mix rises, “it should be responsible for an increasing share of the grid’s burden of reliability, stability and high power quality,” NREL concluded.
Funding for the project was provided by the U.S. Department of Energy’s SunShot Initiative, NREL noted.