A mechanical failure on a 230 kilovolt transmission line in Northern Virginia triggered the largest data center disconnection in PJM's history, according to a preliminary review PJM and Dominion Energy presented at PJM's Aug. 6 Operating Committee meeting. Roughly 3,800 megawatts of data center and other large computational load in the Dominion zone dropped off the grid and switched to backup generation on July 22, PJM said. The system did not see significant reliability impacts, but operators faced a sharp imbalance between generation and load, and large swings in voltage and frequency, before regaining control.
How the fault unfolded
Dominion's preliminary investigation found that at 7:56 a.m. that Wednesday, a mechanical failure automatically removed the 230 kV line from service. The fault cleared correctly, but data centers in the Dominion zone disconnected from the grid anyway, said Brian Starling, director of the system operations center at Dominion Energy. The first wave, 2,970 megawatts, transferred to backup generation and pushed system voltage up. That voltage rise then prompted a second wave of 1,099 megawatts of data centers to disconnect and switch to their own generation, Starling said. In all, about 3,800 megawatts of Large Load transferred away without warning.
The response, by the numbers
| Measure | Figure |
|---|---|
| First load-transfer wave | 2,970 MW |
| Second load-transfer wave | 1,099 MW |
| Total Large Load disconnected | approximately 3,800 MW |
| PJM system load, before and after | 99,984 MW to 96,205 MW |
| Area Control Error recovery time | 9 minutes (NERC limit: 30 minutes) |
PJM's overall system load fell from 99,984 megawatts to 96,205 megawatts as the event unfolded. The PJM Control Center dispatched generation down to close the gap between generation and load. That restored system frequency to 60 hertz and helped hold grid stability. Operators also dispatched reactive power resources to bring voltage back down. PJM recovered its Area Control Error, the standard measure of the gap between generation and load, within nine minutes, inside the 30-minute limit set by the North American Electric Reliability Corporation.
When large blocks of computational load disconnect unexpectedly, PJM said, system frequency rises above 60 hertz and voltage rises with it. Grid operators watch both closely because sustained spikes can damage transmission equipment or trigger further disturbances.
A pattern PJM says is growing
The July 22 event was the largest of its kind in PJM's history, but not the first. Similar sudden Large Load transfers of about 1,500 megawatts each hit the Dominion zone on Feb. 17, 2025, and July 10, 2024. "This was a normally cleared fault," Operating Committee Chair Emanuel Bernabeu said. Bernabeu said data centers should not disconnect from the grid over a normally cleared fault, and that they are too sensitive to voltage changes and are disconnecting too early. He compared the pattern to earlier ride-through problems PJM and other grid operators saw with wind and solar inverters, which stakeholders addressed with voluntary ride-through guidelines in 2019. PJM is now facing a similar ride-through challenge on the load side of the system.
FERC orders new reliability standards
Computational loads such as data centers and crypto-mining facilities are not currently registered with the North American Electric Reliability Corporation and are not subject to its reliability standards. NERC has tentatively defined computational load as load from information technology equipment such as servers, storage and networking hardware. On July 16, the Federal Energy Regulatory Commission ordered NERC to define enforceable reliability standards for computational loads, citing the load growth driven by data centers and the need for more certainty than voluntary timelines provide. FERC set a Dec. 31, 2026 deadline for NERC's first phase of enforcement provisions.
That first phase will not set minimum voltage or frequency ride-through requirements for computational loads, PJM Manager for Reliability Engineering Matthew Wharton said. Those requirements are expected in NERC standards in 2027. PJM's own work on interconnection requirements for Large Load facilities will run through its Planning Committee, said David Souder, PJM's executive director for system operations. "You really need these requirements in place before the load comes on the system," Souder said.
