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A microgrid powered only by solar and batteries points the way to bigger all-renewable grids

Australia’s Horizon Power and partner PXiSE showed they could power a town without generators. The implications for expanding renewable energy are massive.

Jeff St. John
Jeff St. John
9 min read
A microgrid powered only by solar and batteries points the way to bigger all-renewable grids

In May of this year, Onslow, a town of 848 people on the coast of Western Australia, did something no town of its size has ever done before. For 80 minutes on a sunny day, it ran its power grid entirely on solar and battery power.

It’s an accomplishment that could have major implications beyond this remote community. That’s because it indicates that the right combination of technology can support much greater penetrations of clean energy resources than most grids can handle today — and without the fossil-fueled spinning generators that are now vital for maintaining stable grid power.

It’s taken more than two years for utility Horizon Power and technology partner PXiSE, a subsidiary of California-based utility holding company Sempra Energy, to install, test and configure the technology involved in enabling its first “hydrocarbon-off” test of its system, as Horizon Power CIO Ray Achemedei described its May event.

“The weather conditions were perfect: significant levels of solar coupled with low demand” on an Australian winter day, Achemedei said in an interview last week. “That allowed us to start curtailing the natural gas power station” that provides the remote microgrid’s core generation capacity. “One by one, the turbines started to go idle, until the entire power supply of the town was being met by renewables.”

At that point, the Onslow microgrid frequency — the steady sine wave of alternating current that must be maintained at almost perfect fidelity to prevent cascading grid outages — was being “formed” not by the spinning turbines, but by the inverters at its two utility-controlled 1-megawatt battery installations.

That stable frequency was also being matched and reinforced by the 600 kilowatts of utility solar, as well as the rooftop solar power from about 260 residential and eight commercial customers, about 30 of them with behind-the-meter batteries, “all of which accounts for well over 2 megawatts,” he said.

Coordinating those hundreds of distinct inverter-based power systems against the constantly varying demand from the town’s electricity customers was a “technically challenging endeavor,” he said. The PXiSE system that coordinated it uses a novel combination of automated and data-responsive controls at the hundreds of inverters that need to act at the 60-cycles-per-second speed of the grid itself to ensure the stable hum of power across its circuits.

But initial data from the 80-minute trial run suggests that its groundbreaking fossil-fuel-free operations “could have gone longer,” Achemedei said. “This was just a test to see what we could do it.” More tests are planned in the next few months, he said.

“If all goes well, we’ll leave it in that state permanently — the system will automatically go hydrocarbon-off under the right conditions and make its own decisions,” he said. At that point, Horizon Power can start to contemplate switching the other 34 remote microgrids it operates to similar standalone solar and battery operations. This will allow it to start reaping the benefits of massively reduced fuel costs and carbon emissions across its sparsely populated service territory.

“It was an enormously huge moment for us and for our customers,” Achemedei said.

Onslow, Western Australia (Image credit: Horizon Power)

Why an all-inverter-powered grid is such a big deal

It was also an enormous moment for PXiSE CEO Patrick Lee.

“The accomplishment in Onslow is different from anything else done in the world,” Lee said in a June interview. While other communities, or even entire countries such as Costa Rica, can claim 100 percent renewable-energy-powered operations, they rely on spinning generation from hydropower, geothermal power or biodiesel-fueled generators, he said.

Similarly, the microgrids operating around the world all rely on some form of spinning generation at their core, as well as centralized control of generation, storage and loads to maintain stable power quality, he said.

In Onslow, by contrast, “we’re powering a community on 100 percent inverter-based technologies,” with a central battery system that’s “relatively small. The system load is actually bigger than the battery. We’re relying on solar through most of the day, only using the battery to augment the solar.”

This kind of distributed, inverter-based orchestration is what PXiSE (pronounced like the word “spice” without the “s”) was created to do. Since its late 2016 founding, the collaboration between Sempra, OSIsoft and researchers at the University of California at San Diego, with financial backing from Japan’s Mitsui, has grown from testing at Sempra-owned wind farms in Hawaii to managing about 700 megawatts of assets at more than a dozen renewable power sites and microgrids around the world.

PXiSE’s technology is built on high-speed, time-synchronized decoupled feedback control of the disparate systems involved. “We’re actually controlling customer-owned resources as part of the overall [distributed energy resource management system]”, he said, “and also using microgrid technology at the substation to do the overall balancing of the grid, to make sure the generator can be turned off.”

Spinning generators provide inertial stability to grids, acting as shock absorbers to the constant and unpredictable fluctuations in demand and supply that can pull grid frequency and voltage out of safe operating boundaries. In an analogy to the human circulatory system, frequency is like the “heartbeat” of the grid, Lee said, and “that heartbeat cannot skip — it has to be there all the time. We have to switch the heartbeat from the generator to the battery.”

“The other part is the blood pressure — the voltage of the system,” he said. “If that blood pressure is too low, the solar that the customers have will trip offline. Not only do you need to synchronize the heartbeat, you need to keep the blood pressure high enough so that you can properly function.”

Spinning generators have always served as the central source of both frequency and voltage of the world’s power grids. While isolated systems can operate solely on inverter-based solar and battery systems without too much trouble, getting multiple interconnected systems to maintain frequency and voltage in tandem requires careful programming of some inverters to serve as “grid-forming” inverters — those that set the “heartbeat” of the grid, so to speak — while others carry on in their traditional “grid-following” role of adjusting to what’s happening on the grid they’re connected to.

“PXiSE has the technology to manage voltage and frequency simultaneously, in a single move,” Lee explained. Other technologies out there “take two moves — and in between moves, that can cause coordination problems.”

Inverters have a lot of flexibility to help balance both frequency and voltage in real time, but coordinating those activities as a network is a very complicated task. PXiSE uses the massive streams of sub-second data from phasor measurement units to coordinate the time and location-dependent variables that inform how these orchestrated inverter operations must be carried out. The graph below, taken from a demonstration of its technology on Guam, shows how the system was able to stabilize frequency by modulating real power from the Pacific island's 25-megawatt solar farm when it was turned on versus periods before and after when it was inactive.

Image credit: PXiSE

That coordination has benefits beyond being able to turn off generators, Lee noted. During everyday operations, PXiSE’s distributed energy resource management system [DERMS] “dispatches resources with respect to the network’s operating limits,” he said. That allows “maximum hosting of [distributed energy resources] and renewables,” with an overview of how the distribution network is configured at any point in time to “adjust the real power and the reactive power of each device on the network.”

That’s allowed Horizon Power to dramatically expand how much solar it’s able to host on its Onslow system since it turned on PXiSE’s DERMS capabilities in spring 2020, Achemedei said.

“We’re running comparatively around three and a half times the amount of solar penetration you’d allow on a grid of that size,” he said. Without that DERMS...in place, Horizon would have faced the risk of rising levels of solar introducing destabilizing factors like reverse power flows — power running from customers’ homes back up the circuits that serve them, which can trip protective grid equipment if left unaddressed — or clouds passing overhead that can cause quick and dramatic drops in solar production.

“Without significant further levels of investment in infrastructure and network augmentation, you run the significant risk of outages,” Achemedei said.

The game-changing impact of technology that can enable all-renewable grids

The types of capabilities that Horizon and PXiSE have been testing out in Onslow are becoming a high priority for utilities and grid operators around the world. Rising levels of wind and solar power are supplanting central spinning generation as a primary source of grid electricity, from wind power in Australia and Northern Europe to utility-scale and rooftop solar in U.S. markets such Hawaii, California and Arizona.

Australia is characterized by sparse and sometimes fragile grids that are being required to absorb fast-rising levels of renewable energy. The country has been a primary testing ground for this work, with notable projects including Dalrymple ESCRI in South Australia demonstrating the potential for grid-forming inverters to manage grids powered entirely by wind farms.

“That’s indicative of the kinds of things we’re going to be seeing in wider grids in the future,” said Aidan Tuohy, program manager of bulk system renewable integration at the U.S.-utility-funded Electric Power Research Institute (EPRI). “During parts of the year, they may be serving an increasing portion of their loads with renewables and batteries.”

EPRI has multiple projects underway to test these kinds of capabilities, including the Solace project in Austin, Texas, which is funded by the U.S. Department of Energy. Handling grids that are mostly or entirely powered by inverter-based renewables and batteries is “more straightforward when you’ve got four or five resources,” Tuohy said.

“But as you get to larger systems, you have to make sure these grid-forming resources can operate with each other and behave as they ought to behave in different circumstances,” such as extreme weather events.

Achemedei highlighted the complexity of standing up Horizon’s Onslow project, which got started in early 2018.

“It’s fair to say there’s nothing here that’s out of the box,” he said. Getting the secure communications and control architecture up and running was a challenge — Horizon used secure gateway devices from Australian vendor SwitchDIN for that task — as was managing the “data-hungry” use of synchrophasor readings to inform the PXiSE platform.

The technology standards for inverters to take in, respond to and communicate their actions under the PXiSE control schema are also just starting to be implemented by inverter vendors, he added. “There are literally only two inverters that are currently able to be utilized” for the Onslow project, Achemedei noted.

Lee agreed that the industry is still catching up to supporting the key standard in question, IEEE 2030.5 — the primary standard for enabling “smart inverter” functions being demanded by major solar markets including California.

In fact, PXiSE’s servers are the first to be certified to that standard, in terms of serving as a central control platform for advanced inverter functions, he said. California regulators have approved a request from Sempra utility San Diego Gas & Electric to deploy PXiSE’s technology at several microgrid pilot projects, subject to audits to ensure that the two companies owned by the same parent do not use that relationship in ways that could present conflicts of interest with independent microgrid developers.*

Image credit: PXiSE

Getting more IEEE 2030.5-certified inverters into the field will be an important step for enabling PXiSE’s DERMS capabilities across a much broader range of potential markets, he said. So will the process of creating standards for how utilities orchestrate communications and control across “hundreds of thousands of different devices at scale,” Lee said.

Then there are the utility customers to consider. Controlling solar to balance the grid will require curtailing that solar output at times, or asking customer-sited inverters to reduce their real power supply to the grid. Onslow’s system relies on curtailing utility generation before customer generation, Achimedei said — but it may be more efficient for the system to curtail customer generation at times, particularly if that curtailment allows more customers without solar to install it, he noted.

“How much are your customers prepared to accept curtailment of their [distributed energy resources] throughout the year?” he said. “Our relationship with customers around solar is changing.”

*Correction, 14 July 2021: An earlier version of this article stated that California regulators were considering a request to deploy PXiSE’s technology in microgrid projects. In fact, the California Public Utilities Commission provisionally approved the request in 2020.

(Lead photo: Horizon Power's utility-scale solar and battery farm, part of its Onslow microgrid. Photo courtesy of Horizon Power)

microgridsolar powerenergy storagerenewable energy

Jeff St. John

Jeff St. John covers technology, economic and regulatory issues influencing the global transition to low-carbon energy. He is former managing editor and senior grid edge editor of Greentech Media.