In most large power systems, transmission grid operators are “still able to maintain grid reliability using synchronous generators present in their systems at this time,” notes a report from the Energy Systems Integration Group.
But with “ever-increasing levels” of inverter-based solar, wind, and battery storage, “early proactive deployment” of grid-forming (GFM) resources “can mitigate reliability challenges that could otherwise require significant transmission infrastructure investment,” says the report, prepared by ESIG’s GFM Testing Project Team.
GFM capabilities, which “can be enabled relatively easily through software changes,” would be especially valuable for the hundreds of gigawatts of battery storage capacity in interconnection queues, the authors say.
ESIG previously published a study showing that large-scale batteries with grid-forming inverters can increase renewables hosting capacity.
Proactive
The midcontinent grid operator MISO has developed GFM specifications that will apply to all future transmission-connected battery energy storage systems, the study notes.
And Texas grid operator ERCOT has a proposal undergoing stakeholder review that would require GFM for transmission-connected BESS projects with interconnection agreements signed after April 1, 2026.
Testing
The ESIG report, titled “Testing the Performance of Grid-Forming Resources,” focuses on test methods and performance metrics for evaluating voltage source behavior, which it terms “the essence of a GFM resource.”
In other words, “the unique characteristics of a GFM resource are the fast voltage- and frequency-stabilizing response during the short time frame following a grid disturbance, and the ability to act as a near-ideal voltage source with an internal impedance in these fast timescales.”
The report presents test methods that “could be used in specifications that are being developed for GFM resources,” such as specifications from the UNIFI Consortium.
The test methods for evaluating the core capabilities of GFM resources focus on performance metrics and pass/fail transient tests.
The performance metrics are defined as time-domain or frequency-domain specifications. The pass/fail transient tests focus on certain abnormal grid conditions such as the operation of a GFM resource during the loss of the last synchronous generator on the system or under extremely weak grid conditions.
The report suggests several sets of performance metrics, but advises that the metrics should be adapted based on the system characteristics where a GFM resource is going to be installed and on quantifiable objectives for improving system strength and stability.
ESIG describes itself as a nonprofit organization that “marshals the expertise of the electricity industry’s technical community to support grid transformation and energy systems integration and operation.”
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