New advanced inverter technology is already proving it will help bring more distributed solar more safely to the grid.
Concern with, and sometimes resistance to, increasing amounts of distributed solar often comes down to how solar-generated electricity flowing IN to the distribution system will affect the lines that have traditionally fed power OUT to consumers.
Smart technology that protects feeder systems, only theoretical until recently, has been installed and is now being demonstrated by two teams of utility, academic, and solar industry researchers.
The Projects
A $4.4 million DOE SunShot High Solar Penetration initiative grant and $2.7 million contributed by the inverter maker Solectria Renewables and utility partners DTE Energy, National Grid, and Pepco Holdings is funding one project.
The Electric Power Research Institute (EPRI)-led group is readying real world demonstrations at three large solar installations. DTE has installed an approximately 350 kilowatt inverter at its site. National Grid has two inverters of approximately 500 kilowatts running at one site in its territory and two of about 380 kilowatts at another. Pepco is working at a single site with eighteen inverters of about 100 kilowatts each.
“The size and number of smart inverters is dependent on the size of the solar,” explained EPRI Technical Executive Brian Seal. All use the same technology and architecture.
The University of Hawaii’s Natural Energy Institute (HNEI) is leading a smart inverter demonstration focused on residential feeders. It is funded with a $6.1 million Department of Energy SunShot High Solar Penetration initiative grant. A further $6.1 million comes from smart communications pioneers Silver Spring Networks (SSN), Exegin, and inverter makers Fronius, Hitachi, and SMA, as well as from utility partners Maui Electric and Pepco.
A Fronius inverter is installed with a residential solar system in Maui and a Hitachi device is in place with another in Pepco’s territory, HNEI Project Manager James Rawson said. Each is equipped with SSN and Exegin communications that allow the utilities to control them through AMI infrastructure built through a separate DOE smart grid initiative. Eventually, the HNEI group will install 30 smart inverters in Maui and 10 in the Pepco territory.
In both programs, advanced inverters have already shown they can connect to the grid and open two-way communication with utility control centers. Within the next year, they will show exactly how much protection they can give feeder lines.
Maui Electric and Pepco have been extremely interested and engaged and valuable partners, Rawson said. “We all hear about utility resistance to renewables but that is far from what we’ve seen on this project.”
The feeders
Concerns with distributed energy resources differ, depending on the lengths, architectures, and voltages carried on the feeders, Seal said. But the concerns largely fall into three categories. First, there can be too much voltage. Or the amount of voltage can change too fast.
“Voltage can vary significantly, and suddenly, like when a large solar installation goes from full sun at one moment to a heavy cloud cover the next,” he said. “The feeder may be able to handle that amount of power but not that much change.”
Third, the power can be flowing in and out at the same time. “Feeders were designed for power to flow out to customers at the ends of the lines. But solar flows in at the feeders,” Seal said. “That can disrupt the settings of the protection devices.” If system protective devices fault out, it can lead to distribution and transmission system equipment damage and, eventually, bigger trouble.
Traditional generation, and large solar power plants similarly interconnected to the transmission system through substations, do not cause overvoltage or bidirectional energy flow concerns. But large solar plants can, Seal noted, have problematic voltage fluctuations from the sun’s variability.
EPRI’s detailed analysis of the hosting capacity of feeders helped shape Common Functions for Smart Inverters, which describes eighteen theoretical ways smart inverters can increase the grid’s hosting capacity for distributed energy resources. The DOE projects will demonstrate that “a real smart inverter on a real power system,” Seal said, can “raise the level of solar generation that can safely be accommodated.” This is increasingly important because the level of solar coming into the system, Seal added, “is no longer insignificant.”
Early results
The EPRI group’s research has shown that smart inverters do not significantly add cost. “Solar inverters tend to be very capable devices, almost PC-like in terms of their speed and power and memory,” Seal said. “It is mostly a matter of programming and sending and receiving in a way that is compatible with the utility’s operation’s center.”
There is “a very high degree of confidence” in the technology, Seal said, noting that California’s Rule 21 covering grid connection is being revised to require smart inverter functionality for solar. And it “speaks volumes” about smart inverter potential that engineers from the different utilities are working collaboratively.
“Utilities are the ultimate customers,” explained DOE Solar Energy Technologies Chief Scientist Dr. Ranga Pitchumani. “Many want to be part of the solution.”
“Change can happen to you or you can be part of the change,” DOE Solar Energy Technologies Office Director Minh Le added. “Forward-leaning utilities are embracing the change.”
