Nothing To Fear for High-Renewable Systems: NREL Shows Scalable, Resilient, and Secure Systems With Communication-Less Controls
Not long ago, 100% renewable energy was a distinction reserved for remote communities avoiding costly energy imports. But now, some U.S. states are reaching very high levels of renewable energy, and the largest urban areas on the planet are targeting 100% renewable operations, basing their futures around variable power from wind and solar tied to energy storage.
On its own campus, the National Renewable Energy Laboratory (NREL) is also demonstrating 100% renewable operations, if involuntarily, and its message from the experience is that transitioning to renewables is achievable. In fact, NREL has shown that relatively simple controls can enable power grids to operate with 100% wind, solar, and storage, without the need for dedicated device-to-device communications.
"When the Flatirons Campus lost power, we didn't have a microgrid controller capable to black start and manage all the microgrid assets, and building a controller from scratch was impractical with such short notice," said Przemyslaw Koralewicz, an NREL researcher who helped repower the campus during an outage in 2020. "Instead, we developed a communication-less scheme that took advantage of standard frequency controls of the renewable assets and that could be programmed expediently without significant oversight or customization."
Koralewicz and a few other researchers at the Flatirons Campus black started the site initially with a battery and solar array, and soon after connected wind for a total generation capacity of around 2 MW. Their control approach was unique for its simplicity and scalability: Rather than have a central controller issuing commands to the solar and wind assets, each device was programmed independently to produce a sort of self-organizing stability. Real results are shown in the graphic below.
By allowing the frequency to drift between 59.5 and 60.5 Hz rather than trying to achieve a fixed 60 Hz as in typical grids, and by adjusting the devices' generic controls to enable changes in active power based on grid frequency (known as droop control), the NREL team configured the microgrid to be autonomously responsive to any changes, such as a large variation in wind or solar production during turbulent weather conditions, or a new asset like a diesel generator added to the system.
"There are plenty of advantages to running a communication-less system like this," Koralewicz explained. "For one, the cybersecurity threats directed at distributed resources become effectively zero, because data exchange between devices isn't necessary. Additionally, the approach is plug and play for devices, such that renewable assets can be added or removed more-or-less seamlessly."
Once the fun was over and the Flatirons Campus power system had been repaired, the same NREL team pressed ahead with their newfound solution. They asked whether the method could truly work on a larger energy system, with multiple battery storage systems and generation resources, at or near 100% renewable operation. The team demonstrated on a simulated system that their approach remains functional and maintains stability throughout a variety of operational scenarios ranging from 20% to 300% renewable levels and with varying battery capacities and sizes.
In a sense, this method unleashes the flexibility of frequency in electrical systems. Although conventional generation locks in the frequency to a very tight window around 60 Hz, upcoming proposals for the power grid suggest using a slightly wider range of frequency, afforded by the capabilities of modern inverter-based resources. The DOE has launched a consortium named Universal Interoperability for Grid-Forming Inverters (UNIFI) to advance such strategies and develop data and standards to support high-renewable systems.
Apart from the success of the control approach, NREL has shown that even a simple improvised method can serve reliable, renewable power during a resilience event. This is a good indication for communities working toward record levels of renewables, suggesting that some aspects of the transition do not have to be too complicated. NREL has the capabilities to help partners prove solutions for tomorrow's energy systems.
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