New control strategy for grid-forming inverters

Researchers in the United States have created a droop control strategy for grid-forming inverters that purportedly improves power system frequency stability. By using an exponential active power–frequency relationship, the novel technique optimizes the use of available headroom, reduces frequency deviations, and enhances overall grid resilience.

October 1, 2025 Emiliano Bellini

Image: ImagePerson, CC BY 4.0 via Wikimedia Commons

Renewable energy resources using grid-forming inverters can actively regulate voltage and frequency in the electricity grid, mimicking the inertia of synchronous generators. This capability enhances the frequency stability and response of power systems.

With this in mind, a research group from the University of Colorado Boulder in the United States has developed a droop control technique designed to improve the disturbance response of power systems of any size, particularly those with a high share of grid-forming inverters.

Droop control works by replicating the inherent “droop” characteristics of traditional synchronous generators, ensuring that output power is inversely related to system frequency for active power, and to voltage for reactive power.

The novel control strategy, called Droop-e, establishes a non-linear active power–frequency relationship using an exponential function of power output. According to the researchers, this approach enhances the utilization of available headroom while reducing frequency excursions and the rate of change of frequency.

In this context, headroom refers to the grid’s available capacity to meet future demand without requiring network enhancements.

“The primary idea behind the Droop-e concept is replacing the linear droop frequency control of a grid-forming device with an exponential function of power dispatch,” the scientists explained. “This allows a design in which frequency changes are reduced for devices with large available headroom, enabling them to exchange relatively larger active powers with the network while causing correspondingly smaller frequency deviations.”

The Droop-e controller operates across the full charge and discharge capabilities of a battery energy storage system (BESS). The frequency-setting capabilities of grid-forming inverters create conditions for a secondary controller to make small frequency adjustments and detect the resulting changes in power output.

Furthermore, the controller’s exponential response allows the grid-forming device’s frequency to adjust and eventually achieve a “parameterized” power-sharing objective.

The researchers reported that curve continuity, synchronization criteria, and small-signal stability analysis all confirmed the stability of the proposed controller. It reportedly achieved greater utilization of available headroom, a less deviant frequency nadir—the lowest point of a network’s frequency drop during periods of imbalance—and a more favorable rate of change of frequency, along with improved frequency dynamics and natural power-limiting behavior.

“The Droop-e control strategy shows immense potential for improving the frequency stability and resilience of emerging power grids by utilizing available energy resources more efficiently, particularly BESS that are typically not operating near their power limits,” the team concluded.

The new technique was introduced in “Autonomous grid-forming inverter exponential droop control for improved frequency stability,” published in the International Journal of Electrical Power & Energy Systems.

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