In short. An automatic synchroniser adjusts the incoming source until voltage, frequency and angle match, then issues a close command timed to allow for breaker closing time. An independent check-sync relay confirms the window, so no single failure can close out of phase.
The sequence
Incoming source running → permissives OK → speed and voltage matching → slip and angle in window → close command → breaker closes → feedback → load take-up.
Matching variables
| Variable | Typical control | Why |
|---|---|---|
| Frequency (slip) | Speed raise and lower pulses or analogue bias | Slow slip gives time to close at zero angle |
| Voltage | AVR raise and lower or analogue bias | Limits reactive shock |
| Phase angle | Close command leads the angle by closing time | Contacts meet near zero angle |
| Phase sequence | Checked before first operation | Wrong sequence is a short circuit |
Two layers of protection
- The automatic synchroniser decides when to close.
- An independent check-sync relay permits the close only inside a narrow window of voltage, frequency and angle. Wire it in series with the close circuit. See dead-bus and sync-check logic.
Timeouts and failure cases
- Sync timeout: stop and alarm if the window is not reached.
- Breaker fails to close: retry limit, then alarm.
- Close without feedback: trip logic.
- Loss of voltage sensing: block closing.
- Both sources dead: use dead-bus logic with lockout.
Commissioning
- Measure real breaker closing time and enter it.
- Check the windows with a test set or a safe live check.
- Record voltage, slip and angle at closing across several attempts.
See synchronisation and synchronising studies.
Illustrative example
A synchroniser with a 3 percent voltage window and a slip of 0.1 Hz closes the breaker at angles within a few degrees. During test, the check-sync relay is set tighter than the synchroniser to ensure it blocks any close outside its window, and a forced out-of-window attempt is correctly blocked.
What the synchroniser does
An automatic synchroniser adjusts the incoming generator’s speed and voltage until it matches the bus, then issues a breaker close command at the right instant.
| Stage | Action |
|---|---|
| Check sequence | Phase sequence correct, voltages present |
| Match voltage | Adjust AVR setpoint |
| Match frequency | Send raise/lower pulses or analogue bias to governor |
| Match phase | Slip frequency controlled to a slow rate |
| Close | Anticipate breaker closing time and close slightly before zero phase angle |
Advance angle = 360° × slip frequency × breaker closing time
With 0.1 Hz slip and an 80 ms breaker: 360 × 0.1 × 0.08 = 2.9 degrees. The synchroniser issues the close command about 3 degrees early so the contacts touch at zero.
Typical acceptance windows
- Voltage difference: within about 5 percent.
- Frequency difference: within about 0.1 Hz.
- Angle: within about 10 degrees at close.
Independent backup
A separate sync-check relay should supervise the close command, so a synchroniser fault cannot close out of phase. See sync-check logic.
Faults
Failure to synchronise often comes from governor hunting, noisy speed signal or wrong voltage transformer phase. See synchronisation.
The master engineering question
If the synchroniser, a sensor or the breaker behaves unexpectedly, does an independent device still stop an out-of-phase closure?
What to send us
Send breaker data and the synchronising scheme. Send your application details.
Questions engineers ask
Why compensate for breaker time?
The breaker closes some tens of milliseconds after the command, and the angle keeps moving.
Is manual synchronising still used?
Sometimes as a backup, with a synchroscope and check relay.
What if slip is too fast?
The synchroniser slows the incoming set or waits for a better window.
Do I need check-sync with an auto synchroniser?
Yes, for independent protection.
Values are illustrative. Follow manufacturer settings.