By Mahesh Morjaria, Vladimir Chadliev, Vahan Gevorgian and Clyde Loutan
A key enabler in integrating large amount of PV generation into the electric power grid, is the capability of utility-scale PV plants to address grid reliability and stability concerns. PV plants with “grid-friendly” features such as voltage regulation, active power controls, ramp rate controls, fault ride-through, frequency droop control and others have alleviated these concerns.
The viability of PV plants to provide important ancillary services to the grid was recently demonstrated in a test conducted with NREL and CAISO on a 300MW utility-scale PV plant. The results showed that the PV plant value can be extended from being simply an energy source to provide services such as spinning reserves, load following, ramping, frequency response, variability smoothing & frequency regulation. The results showed that a PV plant can regulate to 4-second Automated Generator Control (AGC) signal 24-30 points more accurately than even fast gas turbines.
With an increasing share of variable generation on the grid, traditional power generation resources equipped with automatic generation control and automatic voltage regulation controls are being displaced. To support grid stability and reliability, deployment of PV power plants that incorporate advanced grid services capability becomes increasingly essential [1-4]. Recognizing this need, CAISO with support from NREL & First Solar, ran a set of tests on a 300MW utilityscale PV plant to measure its ability to provide ancillary services to the grid.
“Deployment of PV power plants that incorporate advanced grid services capability is becoming increasingly essential.”
CAISO published the test results quantifying the performance of a utility-scale PV plant to provide services that range from spinning reserves, load following, voltage support, ramping, frequency response, variability smoothing and frequency regulation to power quality . Specifically, the tests conducted included various forms of active power controls such as automatic generation control (AGC) and frequency regulation, and droop response, as well as reactive power/voltage/power factor controls.
As an example, Figure 1 demonstrates the plant’s nearly perfect ability to follow the CAISO’s four-second AGC dispatch signal within its selected regulation range of 30 MW, or 10 percent of rated plant power. The plant was commanded to curtail its production to a lower level (red trace) that was 30 MW below its available power. The AGC signal was then fed to the power plant controller (blue trace), so the plant output (orange trace) was changing following the set point while the plant output was increasing during the morning period.
The Figure 2 shows the plant’s behavior during the midday period. In both the figures it is quite clear that the plant’s response is very close to the AGC signal demonstrating high level of performance.
CAISO measures the accuracy of a resource’s response to EMS signals during 15-minute intervals by calculating the ratio between the sum of total 4-second set point deviations and the sum of AGC set points. As shown in Figure 3, the solar PV resource performed with much higher accuracy than conventional resources in following 4-second regulation dispatch signals − about 24-30 % points better than fast gas turbine technologies. This performance is a reflection of the underlying fast responding power electronics technology deployed in PV inverters along with well-designed power plant controllers. The data from these tests will be used by the CAISO in future ancillary service market design for determining the resource-specific expected mileage for the purposes of awarding Regulation Up and Regulation Down capacity.
There has been considerable discussion on the capabilities of wind and solar plants to provide ancillary services and a call to action for all new resources to have capabilities to support grid reliability. And to great extent, these changes are well underway. The results described here clearly demonstrate that utility-scale PV plants are not only capable of providing essential reliability services that can enhance system flexibility and reliability without incurring carbon emissions but can perform even better than conventional plants. With ever decreasing cost of solar electricity, the opportunity cost associated with some curtailment to provide services like up regulation will reduce as well.
Mahesh Morjaria is vice president of PV Systems Development at First Solar. Vladimir Chadliev is director of Global Grid Integration at First Solar. Vahan Gevorgian is principal engineer at the National Renewable Energy Laboratory. Clyde Loutan is principal, Renewable Energy Integration at California ISO.