Synchronism Check Relay Testing: Voltage Window, Angle Window and Slip

Synchronism check relay testing proves that a 25 element closes the breaker only when the two voltages are within the set voltage, angle, and slip limits. It verifies the live-bus and dead-bus logic, the close pulse, and the blocking of an out-of-window closure.

Key takeaways

• Two voltages, one reference. Use UA for one bus and the independent UX channel for the other; connect the neutrals to a common reference.
• Test the window, not only the center. A relay can close at the center angle and still close outside the window if the setting or the measurement is wrong.
• Slip is a rate. A static angle test does not prove the slip check. Use a slow frequency offset and confirm the close command at the expected angle.
• Dead-bus logic is separate. Test the live-live, live-dead, and dead-live conditions if the scheme uses them.
• Measure the close pulse. The breaker close circuit needs a pulse of the correct width; a continuous closure can damage the mechanism.

What does a synchronism check test prove?

A synchronism check relay permits a breaker closure only when the voltage difference, phase-angle difference, and slip frequency between two buses are within limits. It may also have live-bus and dead-bus logic, which allows a closure when one side is dead and the other is live.

The test proves the voltage window, the angle window, the slip setting, the dead-bus and live-bus thresholds, the phase-sequence check, and the close pulse. It does not prove the breaker failure protection or the auto-reclosing sequence; test those separately.

How do you test a 25 element?

1. Record the settings. Note the voltage window, angle window, slip frequency, dead-bus and live-bus thresholds, and the close pulse width.
2. Connect two voltages. Use UA for bus 1 and UX for bus 2. Connect UN to the common neutral. Confirm the relay measures both voltages and the phase angle.
3. Test the voltage window. Hold the angle at zero and vary the voltage of one bus. Confirm the close output inside the window and no close outside it.
4. Test the angle window. Hold both voltages at rated and vary the phase angle. Confirm the close output at the center and no close outside the window.
5. Test the slip. Offset the frequency of one voltage by a small amount and confirm that the close command occurs at the expected angle. Repeat with a slip above the setting and confirm no close.
6. Test the dead-bus logic. Set one voltage to zero and confirm the close output if the dead-bus logic is enabled. Repeat with the other bus dead.
7. Measure the close pulse. Record the width of the close output and compare it with the breaker requirement.
8. Check the phase sequence. If the relay uses a phase-sequence check, swap two phases and confirm that the close is blocked.

What settings and test points should you use?

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

The VAE-430 UX channel is independent of the three phase voltages, which makes it suitable for a two-bus sync check test. Confirm that the relay’s two voltage inputs share a common neutral and that the test set does not create a ground loop.

What is the pass criterion?

• Voltage window: within 2 percent of the setting.
• Angle window: within 2 degrees of the setting.
• Slip: within 0.02 Hz of the setting.
• Dead-bus and live-bus thresholds: within 2 percent of the setting.
• Close pulse: within 10 ms of the setting.
• Phase sequence: close blocked for a swapped sequence.

Why does the relay not close, or close outside the window?

No close: the voltage or angle may be outside the window, the slip may be above the setting, or the dead-bus logic may require a different condition. Check the relay metering and the event log before changing the settings.

Close outside the window: the voltage or angle measurement may be wrong because of a shared neutral, a phase-sequence error, or a test set channel that is not independent. Confirm the two voltages at the relay terminals with a meter.

Close pulse too short: the breaker may not accept the close command. Measure the pulse at the relay output, not at the test set input, and compare it with the breaker requirement.

Frequently asked questions

What is the difference between a sync check relay and a synchroscope?

A synchroscope is a meter that shows the phase difference; an operator uses it to close the breaker manually. A sync check relay is an automatic supervision element that permits or blocks the close command based on voltage, angle, and slip.

Why does the relay not close when both voltages are present?

The angle or slip may be outside the window, the phase sequence may be wrong, or the live-bus logic may not be satisfied. Check the measured angle and slip first.

How do I test the slip setting?

Offset the frequency of one voltage by a known amount and observe the close command as the phase angle rotates. The close should occur within the angle window. Repeat with a slip above the setting and confirm no close.

Can I test a sync check relay with one voltage channel?

No. The element needs two voltages to measure the difference. Use a test set with at least two independent voltage channels, such as the VAE-430 with its UA and UX outputs.

Do I need to test the dead-bus logic?

Yes, if the scheme uses it. A dead-bus closure is a different logic path from a live-live closure, and it has its own voltage threshold.

Where VA-TEK test sets fit

The VAE-430 has three phase voltages plus an independent UX channel, so it can supply both sides of a sync check relay from one test set. Start with the relay protection testing hub and use the relay testing whitepaper for the record sheet.

Related reading: auto-reclosing relay testing for the dead-time and reclaim sequence that uses the sync check element, and protection relay commissioning checklist for the isolation steps.

Standards and references

• IEEE C37.90-2005 — surge withstand capability of relay systems.
• IEC 60255-127:2010 — functional requirements for over/under voltage protection.
• IEEE C37.106-2003 — guide for abnormal frequency protection (slip coordination).
• Relay manufacturer’s technical manual — sync check logic and close pulse.