Underfrequency Load Shedding Testing: Pickup, Time and df/dt Blocking

Underfrequency load shedding testing proves that a frequency relay trips the correct feeder at the set frequency, within the set time, and that its df/dt and undervoltage blocking elements prevent false operation during recoverable disturbances. The test covers pickup, operate time, both blocking thresholds, and stage coordination.

Key takeaways

• Apply realistic voltage and current. Many relays supervise current at 0.06 × In; a voltage-only test can leave the frequency element enabled but the trip path blocked.
• Ramp slower than the df/dt setting. A 0.5 Hz/s ramp stays below a typical 4 Hz/s blocking threshold, so the frequency element can be measured without the blocking element taking over.
• Start the timer at measured pickup. Operate time is measured from the relay’s own pickup to its trip contact, not from the start of the ramp.
• Test the blocking elements in both directions. A blocking function that never operates is as dangerous as one that always operates.

What does a UFLS test prove?

An underfrequency load shedding scheme is the last automatic defence against a system-wide frequency collapse. It must do three things correctly: trip the selected feeders at the set frequency, wait for the set time so that a transient dip does not shed load, and stay stable when the disturbance is recoverable or when the rate of change is too fast to represent a real generation deficit.

A test that only checks the pickup value proves none of this. The procedure below checks the pickup, the operate time, the df/dt blocking boundary, the undervoltage blocking boundary, and the coordination between stages.

Typical settings and test conditions

The values below are a common starting point for a feeder relay on a 100 V secondary system. Use the approved setting sheet for the site; the test points follow the settings, not this table.

How do you test the frequency pickup?

1. Connect the voltage and current circuits as shown below. Set the source to 50.0 Hz at rated phase voltage and inject at least 0.06 × In current.
2. Reduce the frequency at 0.5 Hz/s. If the relay’s df/dt blocking cannot be disabled, keep the ramp below the blocking setting.
3. Record the frequency at which the trip contact closes. This is the measured pickup.
4. Repeat in the rising direction if the relay has a separate overfrequency stage, or if the scheme uses frequency restoration.

A step method can also be used: reduce the frequency in 0.05 Hz steps and hold each step for the relay delay plus 200 ms. The step method is slower but avoids any ambiguity about the ramp rate. Record the highest frequency at which the relay does not trip and the lowest at which it does.

How do you test the operate time?

Operate time is the interval from the relay’s pickup to the trip contact. The cleanest method is a controlled ramp below the df/dt blocking threshold: the instrument records the f–t trajectory, the relay’s pickup is visible as a change of state, and the trip contact closes at the end of the delay.

1. Start at 50.0 Hz and ramp down at 0.5 Hz/s.
2. Mark the instant the relay picks up (start contact, or the first change in the event log).
3. Measure the interval to the trip contact.
4. Compare with the setting; repeat at least twice.

Do not start the timer at the beginning of the ramp. That measures the ramp time plus the relay delay, and it will always read longer than the setting.

How do you test df/dt blocking?

The df/dt element blocks the frequency stage when the frequency changes faster than the setting. Keep the start frequency above pickup and apply a downward ramp at a rate above the blocking setting. The relay should not trip even after the frequency passes the pickup value. Then reduce the rate until the relay trips.

1. Set the start frequency to 50.0 Hz and the voltage to rated value.
2. Apply a 5.0 Hz/s ramp. Confirm no trip after the frequency has passed the pickup by at least 0.5 Hz.
3. Repeat at 4.9, 4.8 … 3.5 Hz/s in 0.1 Hz/s steps.
4. The highest rate at which the relay still trips is the measured blocking boundary. Compare it with the setting.

Some relays apply df/dt blocking only to falling frequency; others apply it in both directions. Check the device manual and test the direction that is actually enabled.

How do you test undervoltage blocking?

The undervoltage element blocks load shedding when the voltage is too low for a valid frequency measurement, or when a voltage transformer is disconnected. Hold the frequency below pickup and vary the voltage.

1. Set the frequency to 49.0 Hz, below the 49.5 Hz pickup, and the current to 0.5 A.
2. Set the voltage to 68 V line-to-line. Confirm the relay does not trip.
3. Increase the voltage in 0.1 V steps to 72 V. Record the voltage at which the relay trips.
4. Compare with the 70 V setting.

If the relay uses phase-to-neutral settings, convert the test points: 70 V line-to-line is 40.4 V phase-to-neutral on a 100 V system. Applying the wrong base is a common source of a failed blocking test.

How do you wire the VAE-430 for a UFLS test?

The VAE-430 has three current and four voltage outputs. A UFLS test uses one voltage system and one current channel; the independent fourth voltage channel is not required.

Use the 0102 Frequency & df/dt module in PC mode, where the frequency range is 0–1000 Hz with 0.01 Hz resolution and both step and ramp df/dt modes are available. The module records the f–t trajectory and the trip instant, so the pickup and the operate time can be read from one record.

What is the pass criterion?

Use the tolerance declared in the relay manual or the commissioning specification. The values above are typical bench criteria, not a substitute for the site specification.

Why does the relay not trip at the set frequency?

Check the four usual causes before suspecting the relay:

• Current supervision. The injected current is below 0.06 × In, so the relay treats the feeder as unloaded.
• df/dt blocking. The ramp is faster than the blocking setting, so the frequency stage is blocked.
• Undervoltage blocking. The applied voltage is below the blocking threshold.
• Stage not enabled. The stage is out of service, or the trip matrix does not route it to the output being monitored.

Frequently asked questions

Why is the measured operate time longer than the setting?

The timer is usually started at the beginning of the ramp instead of at pickup. Start at the relay’s start contact, or at the first change in the event record.

Can I test UFLS with a single voltage channel?

Yes for the frequency element, but three-phase voltage is recommended because many relays supervise all three voltages and block on voltage transformer failure. Use the same phase arrangement as the site.

Does a UFLS test need current injection?

Many relays supervise current at 0.06 × In. Inject at least that value; 0.5 A on a 5 A relay is a practical test level.

How many stages should be tested?

Every stage in service, including the stage that trips the first feeder and the stage that trips the next. Check the time grading between stages, not only the individual settings.

Why does the relay trip at a frequency below the setting?

The ramp may have passed the pickup before the relay’s measurement window updated, or the relay may apply a fixed additional delay. Repeat with the step method and compare the measured pickup with the setting.

Where VA-TEK test sets fit

The VAE-430 covers the complete UFLS procedure with its 0102 Frequency & df/dt module: 0–1000 Hz in PC mode, 0.01 Hz resolution, step and ramp df/dt modes, and a recorded f–t trajectory with the trip instant. Three current and four voltage outputs are enough for a feeder relay, and the eight binary inputs record trip and reclose contacts directly. For schemes that require six currents — three-winding transformer differential or busbar — the VAE-660 provides six current and six voltage outputs. The test procedure itself is the same; only the number of channels changes.

For the wider commissioning sequence, see the relay protection testing hub and the relay testing whitepaper.

Standards and references

• IEEE C37.106 — Guide for Abnormal Frequency Protection for Power Generating Plants.
• NERC PRC-006 — Automatic Underfrequency Load Shedding.
• IEC 60255-181 — Measuring relays and protection equipment: Functional requirements for frequency protection.

Always test to the approved setting sheet and the relay manufacturer’s instructions. The settings above are examples.

Related guide: auto-reclosing relay testing covers the reclose logic that restores the feeder after the load-shedding stage has operated.