Undervoltage Load Shedding Testing: Voltage, dv/dt Blocking and Time

Undervoltage load shedding testing proves that a UVLS scheme sheds load at the set voltage and time, and blocks on a fast voltage drop or a voltage recovery. It verifies the stage thresholds, time delays, dv/dt blocking, drop-out, and the trip output.

Key takeaways

• Stage logic is the scheme. Test each voltage stage and its time delay separately, then test the stage priority and the trip matrix.
• dv/dt blocking is not optional. A fast voltage collapse caused by a nearby fault should block load shedding; a slow decline should trip.
• Use three voltages. A UVLS scheme may use positive-sequence voltage, three-phase minimum voltage, or a combination. Test the quantity the relay actually uses.
• Check the drop-out. The voltage at which the stage resets and the time it takes to reset are part of the scheme.
• Measure from the voltage step. The time delay starts when the voltage crosses the threshold, not when the test set changes its output.

What does an undervoltage load shedding test prove?

An undervoltage load shedding scheme monitors voltage and trips selected loads when the voltage stays below a threshold for a set time. The aim is to prevent a voltage collapse by reducing load before the system becomes unstable. Many schemes use two or more stages, with lower voltage and longer delay in the later stages.

The test proves that each stage picks up at the correct voltage, times out at the correct delay, blocks when the voltage change is too fast, resets when the voltage recovers, and trips the correct output.

How do you test a UVLS scheme?

1. Record the stage table. Write down the voltage threshold, time delay, dv/dt threshold, drop-out value, and output contact for each stage.
2. Isolate the load trip circuits. Confirm that the trip outputs are disconnected from the actual load breakers or that the test is performed with the loads isolated.
3. Connect a three-phase voltage source. Use UA, UB and UC with UN. Confirm the relay metering shows the expected phase-to-neutral voltages.
4. Apply rated voltage. Confirm that no stage picks up and that the trip outputs are reset.
5. Step to stage 1. Reduce the voltage from rated to the stage 1 threshold in one step. Measure the time from the crossing to the trip output.
6. Step to stage 2. Repeat with the stage 2 voltage and time. Confirm that stage 1 does not operate first if the scheme uses a priority.
7. Test dv/dt blocking. Apply a fast voltage ramp through the threshold and confirm that the stage blocks. Then apply a slow ramp and confirm that it trips.
8. Test the drop-out. Raise the voltage above the drop-out value and confirm that the stage resets and the trip output drops out.

What settings and test points should you use?

How do you wire the VAE-430 to the relay?

If the scheme uses a positive-sequence voltage, a balanced three-phase reduction is the correct test. If it uses a minimum-phase voltage, reduce one phase at a time and confirm which phase the relay selects.

What is the pass criterion?

• Voltage pickup: within 1 percent of the setting, or the relay manual tolerance.
• Time delay: within 5 percent or 30 ms, whichever is larger.
• dv/dt blocking: blocked for a ramp above the setting, trip for a ramp below it.
• Drop-out: reset within the published voltage and time.
• Stage priority: the correct output for the applied stage.
• Trip pulse: within 10 ms of the setting.

Why does the scheme not trip on a slow voltage decline?

dv/dt blocking may be active. If the voltage decline is faster than the blocking threshold, the scheme intentionally blocks. Check the ramp rate and the blocking setting before concluding that the relay has failed.

The stage may be disabled. Confirm that the stage is enabled and that the output matrix maps it to the correct trip contact. A stage can pick up and still produce no output if the matrix is wrong.

The voltage may not have crossed the threshold. The relay may measure positive-sequence voltage, which is higher than a single collapsed phase. Use the relay metering to see the quantity that the element uses.

Frequently asked questions

What is the difference between UVLS and UFLS?

UVLS responds to voltage and is used to prevent voltage collapse. UFLS responds to frequency and is used to arrest a frequency decline. Some schemes use both, with separate stages and time delays.

How do I test dv/dt blocking?

Apply a voltage ramp through the threshold at a rate above the setting and confirm that the stage blocks. Then repeat at a rate below the setting and confirm that it trips. Record the ramp rate, not only the start and end voltages.

Why does the relay trip during voltage recovery?

The stage may have picked up during the dip and timed out after the voltage recovered, or the drop-out setting may be too low. Check the event log for the pickup and the timeout, not only the trip.

Can I test UVLS with a single voltage channel?

Only if the relay uses a single-phase voltage element. For a positive-sequence or three-phase scheme, use a three-phase source and confirm the metering.

Do I need to test every stage?

Yes. Each stage has its own threshold, delay, and output. Testing only stage 1 does not prove stage 2 or the stage priority.

Where VA-TEK test sets fit

The VAE-430 provides three voltage channels and four digital inputs, enough for a two-stage UVLS scheme with separate trip contacts. Start with the relay protection testing hub and use the relay testing whitepaper for the test record.

Related reading: underfrequency load shedding testing for the frequency-based counterpart, and protection relay commissioning checklist for the isolation and record steps.

Standards and references

• IEC 60255-127:2010 — functional requirements for over/under voltage protection.
• IEEE C37.106-2003 — guide for abnormal frequency protection for power generating plants (UFLS coordination).
• IEEE C37.90-2005 — surge withstand capability of relay systems.
• Relay manufacturer’s technical manual — stage table and dv/dt logic.