How the HECO-SolarCity partnership is turning rooftop solar into a grid asset

Can the partnership change how we think about solar interconnection and utility-installer relations?

Conventional wisdom tells us that electric utilities and solar installers are supposed to be rivals. From the utility’s perspective, the more rooftop solar, the less electricity that customers will purchase from the grid. For installers, the more skeptical utilities are about the advantages of distributed energy, the more likely they are to support policies that curtail it. It’s a narrative played out across the nation, from Wisconsin to Arizona and beyond.

But one new partnership hopes to change that paradigm by combining the grid knowledge of a major electric utility with the technology of the nation’s largest solar installer and the resources of a top energy lab. Last month, SolarCity and Hawaiian Electric Co. (HECO) announced they were teaming up with the National Renewable Energy Laboratory (NREL) in Colorado to study how to better integrate rooftop solar onto the grid.

It’s a unique type of research partnership, one that both the utility and industry-leading solar provider hope can change how the entire sector looks at distributed resources.

The roots of the partnership

HECO—soon to be acquired by NextEra energy in a $4.3 billion deal—is a utility with a solar challenge. How can it integrate the nation’s highest penetrations of rooftop PV—providing nearly 20% of generation—onto the grid without reliability issues? It’s a question that troubled Hawaii’s largest utility so much it had to hit pause on solar interconnections on some circuits earlier this year, creating a backlog of thousands of customers and prompting many panel installers to flee the island state.

SolarCity, meanwhile, has a related issue. How can it convince utilities everywhere—often the biggest skeptics to solar integration—that distributed energy is not a liability for reliability, and can actually be an asset on the grid?

It was those complementary challenges that brought the two companies together with NREL, according to Colton Ching, vice president of energy delivery for HECO. He says the utility’s own modeling for what could happen on circuits with high solar penetration had “hit a wall,” and that they needed real-world experience in a lab environment to ensure they could connect more solar safely and without reliability issues. Once the companies connected, they decided to use the NREL facility because it had all the resources to test a number of different grid configurations, solar penetrations, and inverter technologies in a safe environment.  

“We thought it would be the perfect place to take different kinds of inverters and actually subject them to the kind of things you would never want to subject an actual grid and actual components to,” Ching said.

For SolarCity, the partnership had obvious advantages from the start, Peter Rive, co-founder and CFO of SolarCity, said in an interview with Utility Dive.

“The impetus for this was basically that if there are any operations concerns in regards to interconnecting solar we want to address them,” he said. “[HECO] had a very specific overvoltage concern so we got together with NREL and they have a great lab that gives you the ability to simulate these edge grid conditions that HECO was concerned about.”


HECO’s technical problem

Ching and Rive said HECO had immediate concerns about a specific problem with solar integration: Transient overvoltage. What would happen on high-solar circuits when the grid connection for the solar units is suddenly interrupted, like if a tree fell on a power line?

When grid connection is interrupted, Ching explained, “the breaker in the substation suddenly opens up … and now you have these homes with their PV systems and a circuit that’s no longer connected to the grid.”

“What happens to that power that was going back to the grid, what happens when it has nowhere to go?”

HECO feared that the fractions of a second when the switch was open could cause voltage on the circuit to spike dangerously. If that was the case, Ching said, such interruptions could cause safety and reliability issues, potentially damaging utility and customer equipment.

“What our testing set-up in NREL did was literally test that phenomenon,” he said.

Encouraging preliminary results

Testing the impacts of sudden grid disconnection comes down to the performance of inverters on the circuit. The engineers needed to know if they would react fast enough to turn solar systems off in time to avoid harmful voltage levels.

The answer, according to Rive and Ching, is a resounding ‘Yes.’

Whether it was “inverters built for larger commercial systems or smaller residential systems, we saw a consistent pattern where inverters turned themselves off very, very quick—much quicker than we thought,” Ching said.

Engineers from the companies tested a number of typical inverters without special modifications or treatment. The results were so encouraging for HECO that it recently announced a new plan to interconnect the more than 3,000 backlogged solar customers. Rive said the decision came directly from the results of the inverter studies.

“The primary liability concern they had has been addressed,” he said. “Therefore it’s giving them the confidence to increase the amount of solar power systems that they have on certain circuits.”

HECO overgeneration hotspots on Oahu 

Further testing and long-term goals

While the testing so far has aimed to prove that high solar penetrations need not do harm to the grid, further research will aim at proving it can enhance reliability and act as a grid resource, Rive said.

“­Our goal is to show in the long run—and by the long run, I mean next year—how smart inverters can be a massive asset for the utility operations by providing frequency supports by providing VARs and by providing voltage support,” he said.

Rive also indicated the NREL testing would include research on SolarCity curtailment systems and storage technology. Curtailment would come from automated software deployed in the substation that stems the flow of power coming from rooftop systems if the generation to load ratio gets too high.  

By 2020, SolarCity aims to bundle energy storage "by default" with every rooftop solar system it sells, and Rive said the impetus behind that push is the same as what predicated the partnership.

“In general, the big thing is that we want to make these distributed resources a tool for the grid operators and that’s the plan with the battery systems as well,” he said. “­There’s a lot of value to distributed resources and it’s our goal to show all of the utility operators how they can take advantage of it to lower their prices and increase reliability.”

Both Rive and Ching have high expectations for the partnership between the companies and NREL, and hope their model of cooperation can be replicated elsewhere.

“I do see it as a paradigm shift for sure,” Rive said, “and I think we’ll be seeing that shift be happening over the next couple of years, where first we’ll address any reliability issues, and then transition to grid operators saying ‘Hey [solar] is great. I like it and I can use it to make the grid more resilient.’"

Ching says his goal for the project is for it to become “a shining example for Hawaii and also for the industry in general.” He hopes to leverage the partnership so it can be expanded, and more utilities can cooperate with more installers to ensure that solar is an asset, not a liability.

“The best results, the best work, comes from working together,” he said.

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Filed Under: Generation Solar & Renewables Technology Corporate News
Top image credit: Flickr; h080