Grid Frequency Response · Path 1 of 2
Frequency drops. Here's what happens, second by second.
That's how many times the Western Interconnection's frequency has dropped far enough, fast enough, that NERC measured the grid's response to it — in the last five years. Not one reached the point where the grid starts cutting power to customers. You've never heard about any of them, because they worked. Here's what happens in the seconds that decide it.
As demand rises, generators draw more current — and by Lenz's Law, that current creates its own opposing magnetic field inside the generator. That field drags against the spinning turbine, physically slowing it down before any machine or person can react.
Various sensors detect the grid's frequency drifting away from its 60 Hz baseline.
Grid batteries discharge stored power within 100–200 milliseconds. Automated turbine valves swing wide open to let in more steam or water.
Audible alarms flash on the grid operator's screens. Every 2–6 seconds, AGC recalculates the exact size of the deficit and sends corrective signals to regulating reserve — a continuous loop that keeps running all the way through Recovery.
The initial battery and valve response keeps running automatically, holding output steady and keeping frequency from falling any further while the rest of the system prepares its next move.
Frequency stabilizes above the threshold — recovery continues below.
That automatic response is proportional to the remaining deficit — as AGC-commanded backup plants close the gap, 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 ramp up using its spare headroom.
Operators switch on fast-starting emergency peaker plants — drawing on non-spinning reserve3, capacity that can start from offline within about 10 minutes.
New resources close the remaining gap — frequency returns to its scheduled 60 Hz.
Frequency crosses the UFLS threshold.
Automated switches intentionally disconnect sections of the public to drop demand and save the grid from collapsing — within seconds, since secondary control (1–10 min away) can't arrive in time to help.
Each stage trips in ≤14 cycles (~0.23 sec) once its threshold is crossed — but whether a real event reaches Stage 5 depends entirely on how fast frequency is falling.
Up to 31.1% of regional load, WECC Coordinated Plan
Frequency returns to its scheduled 60 Hz — the immediate crisis is over, but the shed customers are still without power.
Once frequency is confirmed back above 59.95 Hz, automatic restoration can begin — no sooner than 30 minutes later, and no faster than 2% of load every 5 minutes. Manual, operator-judgment restoration is generally preferred.
References & Notes