Grid Frequency Response · Path 2 of 2
Frequency climbs. Here's what happens, second by second.
That's how much solar and wind power CAISO deliberately curtailed in 2024, just to keep the Western Interconnection's frequency from climbing too high — 93% of it solar, mostly on mild, sunny spring days when supply ran ahead of demand. Curtailment is the last, slowest lever available. Here's what happens in the seconds before it's ever needed.
As supply outpaces demand, generators draw less current — and by Lenz's Law, less current means less opposing magnetic drag on the spinning turbine. With less resistance holding it back, the turbine speeds up on its own, before any machine or person can react.
Various sensors detect the grid's frequency drifting away from its 60 Hz baseline — the same sensors, whichever direction it drifts.
Grid batteries switch to charging, soaking up the surplus within 100–200 milliseconds. Automated turbine valves ease closed to let in less steam or water.
Audible alarms flash on the grid operator's screens. Every 2–6 seconds, AGC recalculates the exact size of the surplus and sends corrective signals to regulating reserve — telling online generation to back off — a continuous loop that keeps running all the way through Recovery.
The initial battery-charging and valve response keeps running automatically, holding output steady and keeping frequency from climbing any further while the rest of the system prepares its next move.
Frequency stabilizes below the threshold — recovery continues below.
That automatic response is proportional to the remaining surplus — as AGC-commanded generation backs off further, there's less error left to correct, so the initial response recedes on its own.
Grid software commands online, already-synchronized generation — called regulating reserve — to throttle back using its available headroom.
Grid operators issue curtailment orders to solar and wind plants — the same lever CAISO already pulls on sunny spring days1, cutting output at the source rather than adding more.
Curtailment and reduced dispatch close the remaining gap — frequency returns to its scheduled 60 Hz.
Frequency crosses the over-frequency protection threshold.
Automated protection relays disconnect generating units to cut supply and protect equipment from over-speed damage — within seconds, since secondary control (1–10 min away) can't arrive in time to help. Unlike Path 1's UFLS, no customer loses power here — it's generation coming offline, not load being shed.
Over-frequency protection is built backwards from UFLS. Nothing tells a generator when to trip — NERC instead defines a “no trip zone” every unit has to ride through. Below 60.6 Hz it must stay connected indefinitely; above that, the time it is required to hold on collapses.3
0.1 Hz is all that separates “hold on for 30 seconds” from “may disconnect immediately.” And leaving the envelope is permission, not instruction — which is why this branch has no five-stage table the way Path 1's UFLS does.
Frequency returns to its scheduled 60 Hz — the immediate crisis is over, but the tripped generation is still offline.
Once frequency is confirmed stable, tripped units go through the normal startup and grid-synchronization process before resuming output — on their own timeline, not a fixed restoration schedule the way UFLS-shed customers get.
References & Notes