Grid-Forming vs. Grid-Following Inverters

Battery storage, solar and wind connect to the grid through inverters, called inverter-based resources (IBRs). How an inverter is controlled decides how it behaves when the grid is disturbed. There are two families:

Grid-following

A grid-following inverter measures the grid voltage with a phase-locked loop (PLL) and injects current in step with it. It’s a mature, well-understood approach, and it’s how nearly all solar and battery inverters have operated. Modern units still provide grid support, such as ride-through, volt-VAR and frequency response, under IEEE 1547-2018 and UL 1741 SB for distribution-connected resources, and IEEE 2800 for transmission-connected ones. But all of that depends on a stable voltage being there to follow.

Grid-forming

A grid-forming inverter holds an internal voltage of its own, with a magnitude and angle it controls, typically using droop or virtual-synchronous-machine control. When the grid voltage or frequency moves, the difference between the grid and the inverter’s internal voltage drives a response straight away, much as a synchronous generator’s rotor does. That gives the grid something grid-following inverters can’t: a fast, inherent resistance to change, sometimes described as synthetic inertia, and the ability to hold up voltage where there’s nothing else to follow.

How weak is the grid? Short-circuit ratio

The usual first screen is the short-circuit ratio (SCR): the short-circuit capacity at the point of interconnection divided by the plant’s rating.

A point of interconnection with 500 MVA of short-circuit capacity gives an SCR of 5 for a 100 MW plant, but only 2.5 for a 200 MW plant. Grids with an SCR below about 3 are commonly treated as weak. There, grid-following controls can struggle, because the plant’s own current visibly moves the voltage its PLL is trying to track. That’s where grid-forming control, or additional studies and equipment, comes in. The same short-circuit capacity idea underlies the transformer impedance guide, applied here at the transmission level.

Where grid-forming matters

• Weak interconnections, including remote renewable zones and long radial lines.
• Microgrids and islanded operation, including campus microgrids, where there may be no other source to set voltage and frequency.
• Black start, restarting a network from nothing, which only a source that forms its own voltage can do.
• Grids losing synchronous generation, where system operators are beginning to require or reward grid-forming capability on new battery projects.

What grid-forming costs

• Headroom. Responding to a disturbance means delivering power or current the plant wasn’t already producing. That takes margin in the inverter rating, and energy in the battery.
• Limited fault current. Inverters can deliver only a small multiple of their rated current at most, far less than the several-times-rated current a synchronous generator supplies in a fault. Protection schemes that rely on high fault current need checking, which the short-circuit study guide covers.
• Studies. Grid-forming behavior depends on control tuning and interactions with nearby equipment. It’s typically verified with electromagnetic-transient (EMT) simulation rather than only the simpler positive-sequence models used for grid-following plants.
• Evolving requirements. Grid-forming specifications differ between system operators and are still being written, so the exact functions required, and how they’re tested, are a project-specific question.

Common mistakes

• Treating grid-forming as a firmware toggle. It needs rating headroom, energy, tuning and studies.
• Assuming inverter-based plants supply fault current like generators. They don’t, and protection has to account for it.
• Screening only on SCR. It’s a first check, not a stability study.
• Specifying "grid-forming" without the functions. State the required behaviors and how they’ll be tested.

Frequently asked questions