In Adjuntas, Puerto Rico – a mountainside town with two community microgrids that are designed to provide energy resilience – researchers with Oak Ridge National Laboratory (ORNL) are studying how to connect the microgrids and share power in ways that benefit the town and serve as a national model.
With 187 kW of combined solar and about 1.1 MW of storage combined, the $1 million grid-tied community microgrids were designed to be hurricane-proof and to keep electricity flowing for up to 10 days for 14 businesses during a power outage. The microgrids are located on two sides of the town square, a configuration that was less expensive than creating one larger microgrid.
The west microgrid serves a number of businesses, among them a bakery and pizzeria plus a pharmacy that needs power 24/7 to refrigerate medicine. The northeast microgrid, located about 1,000 feet away, serves fewer businesses and generates more solar photovoltaics (PV) that could be shared with the west microgrid, said Max Ferrari, research and development associate at ORNL. In fact, it generates so much solar that it’s sometimes necessary to curtail solar generation.
A technology of the future?
Sharing power between microgrids is seen as a technology of the future. Commonwealth Edison’s Bronzeville Community Microgrid is a much watched project that will create a utility microgrid cluster, in this case by linking with the Illinois Institute of Technology’s (IIT) microgrid.
The Bronzeville microgrid is expected to be finished and providing power before the end of the year, said Jennifer McQuilken, director of corporate communications for Enchanted Rock, which is providing the natural gas generator for the project.
During an outage, IIT’s microgrid and the Bronzeville microgrid can both island from the grid and share power. Or, if necessary, either party can cut back on its nonessential loads and share power with the other. And that’s a big deal in the power world.
In Adjuntas, sharing power would also be a big deal.
Sharing power to increase resilience
“In an emergency scenario, when some generation is affected, the ability to share power would allow the microgrids to run longer,” Ferrari said. The lab’s research has shown that if 50% of the microgrid power is lost in one microgrid, both microgrids could be supported with minimum load shedding – if the two are connected. It’s most likely that the west microgrid would need to do more load shedding because it has larger loads.
The lab's study was based on simulations conducted using 2019 load and generation data. The lab continues to collect data to confirm the study's findings, said Ferrari.
“A lot of the work is load management,” he said. Typically microgrids are designed to support critical loads, but, in this case, the microgrids are busy supporting the day-to-day business loads. The researchers collected data about the loads and found that load shedding will be needed when the microgrids are in island mode. This will likely involve educating business owners and asking them to cut back on their energy usage when the microgrids are in island mode.
The community microgrids aren’t connected to a utility distribution system, and each has a storage system controlled by the storage system’s manufacturer, which wants to be in charge of the storage, Ferrari said.
Orchestrator influences controller’s decisions
The researchers are developing a system that will influence the behavior of the microgrid controls but not take control of them.
To do this, the lab is designing an “orchestrator” that can work with any microgrid controller.
“We don’t want to create a system that only works for one microgrid or controller,” said Ferrari. “We want to interface with all microgrid controllers.” The lab’s orchestrator doesn’t enforce set points on the solar or storage but influences how the microgrid controller makes decisions, he added.
If the west microgrid is receiving more power because it’s exporting solar from the northeast microgrid, the west microgrid controller might see that the storage is being charged. The lab’s “orchestrator” would add more PV or more load within the microgrid, influencing the microgrid controller to decide whether to shed load, for example.
Because the ORNL researchers can’t connect the microgrids via utility infrastructure, they’re looking at using a wire that spans the 1,000 feet between the microgrids.
Connecting either via AC or DC
One option for connecting the two microgrids would be a classic connection, an alternating current (AC) wire that runs between the two facilities, said Ferrari.
Another option – the one the researchers are leaning toward – would be a direct current (DC) interconnection. “What we like about this solution: We don’t need to control the assets, we only need power electronics in between the microgrids and a DC connection – in theory,” said Ferrari.
The complexities of ownership
In addition to studying how to share power between the microgrids, the ORNL researchers are also studying different types of ownership models for connected microgrids.
The answers, they’re finding, aren’t simple.
“With two microgrids, it gets complicated,” said Michael Starke, a power systems research engineer for ORNL. “Who decides when to operate the switch, when to close it, when to engage it? You may need a third-party top layer to help coordinate these things. We’re trying to create a communication framework for how to make it work.”
Each microgrid operator needs access to data about the batteries’ state of charge in order to connect them. If one microgrid operator doesn’t know the state of charge of the other microgrid’s battery, the operator can overfill the battery and accidentally turn the system off.
The importance of communication between connected microgrids
“You need communication protocols and to establish information -sharing with the other entity. That part is exactly what we’re trying to establish in Puerto Rico, with two microgrids with two different batteries,” said Starke. Generally, microgrid operators don’t share information about the state of charge of their batteries, he added.
In spite of the challenges, as the researchers move forward, they’re seeing that connecting microgrids is indeed a big deal.
“We are very excited about this,” said Ferrari. “This is more plug-and-play and will allow microgrids to exchange power.”