For a century the grid's stability came from physics most operators never had to think about: large synchronous generators — spinning turbines in coal, gas, hydro, and nuclear plants — whose heavy rotating mass naturally set the grid's frequency and resisted sudden changes. Solar panels, wind turbines, and batteries do not spin in that way. They produce or store direct current and connect to the alternating-current grid through an inverter, a power-electronic device that synthesizes the AC waveform. Collectively these are called inverter-based resources, and as they displace synchronous machines, the question of how the grid stays stable has moved from rotating steel to software and silicon.

That shift is exactly what the Federal Energy Regulatory Commission set out to address. In an October 2023 final action, the Commission directed the North American Electric Reliability Corporation (NERC) to write new standards for these resources. The order states the scope:

The Federal Energy Regulatory Commission (Commission) is directing the North American Electric Reliability Corporation (NERC), the Commission-certified Electric Reliability Organization, to develop new or modified Reliability Standards that address reliability gaps related to inverter-based resources in the following areas: data sharing; model validation; planning and operational studies; and performance requirements.— Reliability Standards To Address Inverter-Based Resources, FERC (Federal Register), source

Grid-following versus grid-forming

Within the class of inverter-based resources, there are two control philosophies, and the difference is fundamental. A grid-following inverter behaves as a current source that locks onto the grid's existing voltage and frequency. It uses a control loop to measure the waveform another resource is already supplying and injects current in phase with it. This works well as long as something else on the system — historically the synchronous generators — is establishing a strong voltage to follow. But a grid-following inverter cannot, on its own, start a grid from nothing, and in a region with very high inverter penetration and few spinning machines, a fleet of purely grid-following inverters has progressively less of a stable reference to follow.

A grid-forming inverter behaves instead as a voltage source. It actively sets and holds a voltage and frequency reference, providing the kind of stabilizing anchor that synchronous generators provided through their physical inertia — but produced through fast control software rather than rotating mass. Because it forms the reference rather than following it, a grid-forming inverter can support a grid with little or no synchronous generation, contribute to restarting a de-energized system, and respond to disturbances in ways a grid-following inverter cannot. The FERC order's record notes that these newer designs behave differently, observing that grid-forming inverter-based resources have different behavioral responses to disturbances on the grid, which is part of why uniform technical requirements are being developed.

The distinction matters most during the first instants of a disturbance. When a large generator trips offline or a fault hits the system, frequency and voltage move quickly, and the grid needs resources that respond immediately to arrest the swing. Synchronous machines did this automatically: their spinning mass released or absorbed energy as inertia, and their physics opposed sudden change before any control system acted. A grid-following inverter, by design, waits to measure the grid's new state and then adjusts, which introduces a dependence on a stable reference and a control delay. A grid-forming inverter, holding its own internal reference, can respond in the first sub-second to support voltage and frequency and can supply meaningful current into a fault, behaving more like the machine it is replacing. This is why grid-forming control is discussed as a way to provide synthetic or virtual inertia and stronger fault response on a grid losing its rotating mass.

Why the rules are being written now

The reason FERC framed its action around reliability gaps is that the existing standards and models were built for a synchronous grid. The order directs NERC to close gaps in four specific areas: data sharing, so operators have accurate information about the inverter-based resources on their systems; model validation, so the computer models used to study the grid actually match how these resources behave; planning and operational studies, so the grid is analyzed with those resources accounted for; and performance requirements, so the resources behave predictably during disturbances. The order also set a deadline for NERC to submit the necessary new or modified standards, putting the inverter-based-resource rulebook on a defined timeline.

Each of the four areas maps to a concrete problem. Data sharing addresses the fact that grid operators have sometimes lacked basic information about the inverter-based resources connected to their systems, including their control settings. Model validation addresses a string of disturbance events in which inverter-based resources tripped offline in ways the planning models did not predict, because the models did not match the equipment's real behavior; validated models are needed so studies reflect what the hardware actually does. Planning and operational studies extend that to make sure the grid is analyzed with these resources properly represented under a range of conditions. Performance requirements set the behavior the resources must exhibit when the grid is disturbed — most importantly, the expectation that they ride through voltage and frequency excursions rather than disconnecting, since mass simultaneous tripping of inverter-based resources is itself a reliability threat. The order set a deadline for the standards to be submitted, putting the work on a defined schedule.

Grid-forming capability sits inside that performance-requirements question.

As the share of inverter-based resources rises, the system needs some resources to form the grid rather than merely follow it, and battery storage — which can act as a fast, controllable voltage source — is a natural candidate for grid-forming control. None of this makes inverter-based resources inferior or superior to synchronous generation; it makes them different, with behaviors that the grid's standards and models were not originally written to capture. That gap is what FERC's order addresses, and grid-forming inverters are the technology at the center of how a grid dominated by solar, wind, and storage can stay stable without the spinning mass it was built around.