One Generator Trips. Grid Frequency Starts Falling.

When electrical demand suddenly exceeds generation, rotating machines slow and system frequency declines. Protection has seconds—or less—to stop a cascade.

1. Recognize the Power Deficit

A generator or interconnector loss removes active power while demand remains. The mismatch draws kinetic energy from synchronized rotating machines.

2. Measure Frequency Decay

Under-frequency relays track the measured frequency. Modern schemes may also use rate of change to understand how quickly the disturbance is developing.

3. Apply the Planned Threshold

When frequency crosses a coordinated setting for its specified time, the first UFLS stage operates. Settings come from the approved regional program and studies.

4. Disconnect a Selected Load Block

Designated feeder breakers open automatically, reducing demand. The goal is to arrest the decline while preserving as much supplied load as possible.

5. Reassess the Frequency Trajectory

Relays continue monitoring after the first stage. If frequency stabilizes, further shedding is unnecessary; if it keeps falling, protection must continue.

6. Advance Through Coordinated Stages

Additional load blocks trip only as deeper thresholds are reached. Staging avoids one indiscriminate disconnection and supports controlled recovery.

7. Validate the Scheme Before the Event

System studies, relay settings, maintenance and event reviews keep UFLS ready. A last-resort scheme works only when every planned stage coordinates.

Shed in Stages. Arrest the Fall. Preserve the Grid.

UFLS converts a dangerous generation-demand imbalance into controlled load reduction, helping frequency recover and preventing wider system collapse.