Kauai, one of Hawaii’s most remote islands, has some of the lowest electricity rates among U.S. islands because its utility, Kauai Island Utility Cooperative (KIUC), adopted a large share of inverter-based energy sources such as solar and batteries, tells the National Laboratory of the Rockies. As the role of inverters increased, operators began to notice a new kind of grid instability not tied to traditional 60 Hz frequency issues but to energy imbalances that caused oscillations in voltage and frequency. These oscillations can damage equipment and reduce reliability if not addressed.
This challenge reflects a broader shift in grid physics: traditional power systems rely on spinning generators that inherently contribute mechanical inertia, but inverter-based resources do not. Instead, their behavior is defined by control software and power electronics, which react differently in dynamic conditions.
Over three years, researchers from the National Laboratory of the Rockies (NLR), the U.S. Department of Energy (DOE), KIUC, and partners studied the Kauai grid. They used detailed sensor data and high-fidelity electrical models to identify which inverters were triggering the oscillations and why. With the DOE-funded SAPPHIRE project, they developed and tested new inverter control strategies using NLR’s ARIES research platform, including grid-forming controls that let inverters actively hold voltage and frequency instead of just following grid conditions.
When the same generator tripped again in 2023, the new controls prevented the oscillation. Grid-forming inverters provided synthetic inertia and stability comparable to spinning generators, reducing the need for load shedding and making the grid more resilient.
The NLR team also developed a method to measure real-time grid strength and inertia by sending small test signals through the system. This lets operators estimate the contribution of electronic and mechanical resources to stability. Findings from Kauai are now informing broader utility planning and efforts to standardize how grid-forming inverters are used to ensure reliable, affordable power.