When it comes to generation and transmission system management, utility-scale solar photovoltaic (PV) arrays are often spoken of as a problem rather than as a solution. However, today’s sophisticated inverters and plant controls are helping to turn around this negative reputation. In fact, results from a recent study show these green-power producers have the potential to provide vital grid-support services as well as any conventional fossil-fuel-fired generating station.
Definitions vary, but the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) uses a 5-megawatt (MW) threshold in its use of “utility-scale.” As an intermittent resource, utility-scale PV systems now can require grid managers to develop complicated contingency plans to handle morning ramp-ups, midday overproduction and end-of-day power declines, along with ability to address the transient effects of passing clouds.
In California, the independent system operator (CAISO) charged with managing statewide grid operations has been a leader in identifying challenges and options related to incorporating solar assets on a large scale. This is no surprise, given the high penetration of solar generation statewide—more than 9,000 MW of transmission-connected PV and an additional 4,000–5,000 MW anticipated by 2020. The organization’s duck-curve graph of power production over the course of a day has become a familiar presence in discussions regarding the operational difficulties posed by widespread utility-scale solar. It charts the roller-coaster ramping up, down and back up again traditional generation plants in their system would be required to make to address shifting solar supplies.
Recently, CAISO spearheaded an effort to see if larger PV arrays could become part of a solution to support more dynamic grid operations. The results are promising. Partnering with NREL and First Solar, a manufacturer/owner/operator of utility-scale PV arrays, the organization put a 300-MW solar farm through its paces to test its ability to respond to a range of voltage and frequency variations and provide rapid-response grid-support services during such conditions.
“We wanted to show these bigger plants could provide these services,” said Clyde Loutan, senior advisor for renewable integration, CAISO. “If we could do it here, we could do it anywhere.”
As described in a January report, “Using Renewables to Operate a Low-Carbon Grid: Demonstration of Advanced Reliability Services from a Utility-Scale Solar PV Plant,” the team developed scenarios that were run on a single day in August 2016 at a First Solar plant in the CAISO system. These included up and down regulation tests during sunrise, midday and sunset; frequency-response tests for over- and under-frequency conditions (i.e., times when power supplies on the connected grid might suddenly rise or drop); tests verifying the plant’s ability to decrease or increase output at a specific ramp rate; voltage- and reactive-power-control tests; and voltage control tests at near-zero active-power levels after sundown.
While these tests might seem esoteric, the real-world implications of the results are not. In each case, the plant responded in line with CAISO expectations.
“It could do anything the conventional plant can do,” Loutan said of the results’ significance. “It was the largest test of its kind in the world.”
Boiled down, these capabilities mean that, with the help of currently existing—and, in many cases, already installed—smart inverters and power plant controls, solar farms can provide new support to transmission systems. This comes just as the need for such support is on the rise, thanks to the growth in solar power production being driven by falling PV panel prices.
Many of these functions have been tested or even adopted by other utility-scale solar plants, though none had been evaluated in such an organized fashion in conjunction with the other capabilities. Because similar equipment also is used in wind farms, the project’s participants are hoping to conduct additional tests at a wind facility to determine if those resources could provide the same kind of grid support.
Understanding the full range of support services solar and wind plants could offer is important information for grid operators. It also could provide benefits to plant owners. With appropriate regulatory support, these services also could provide a new revenue stream by allowing them to bid into ancillary-service markets. Among the next steps outlined in the report’s conclusion are several items targeting the evaluation of federal and state regulatory policies that could be standing in the way of such transactions.
It might seem a stretch to put such emphasis on the results from tests at a single plant on a single day. Anne Gonzales, a CAISO senior public information officer, said the fact that the inverters and controls at the heart of those results are in such widespread use justifies optimism that similar performance capabilities could be common across the installed solar PV market.
“The technology is there for every plant,” she said. “The question is whether every plant has the technology and whether the operators know about it.”