Generator Synchronising Readiness Checklist

In short. Closing a generator breaker onto a live bus out of step can damage the engine, alternator and switchgear. Before synchronising, the incoming unit must match voltage, frequency, phase angle and phase sequence, and the controls, protection and breaker must be ready. This guide gives the readiness questions in order.

The four synchronising conditions

Tolerances come from the manufacturer and the control settings. Do not close outside them.

Readiness checklist

Mechanical and electrical

• Engine runs stably, at rated speed, with no active alarms.
• Alternator voltage is stable, and AVR is in auto.
• Breaker is healthy: spring charged, trip and close circuits supervised, DC supply good.
• Phase rotation of all sources is checked and matches.

Controls

• Synchroniser or check-sync relay is healthy, with correct settings.
• Load-sharing and governor controls are wired and in the intended mode.
• Dead-bus and live-bus logic is correct. See dead-bus, live-bus and sync-check logic.
• Interlocks stop closing onto an unsuitable bus state.

Protection

• Generator protection relays are healthy and set. See generator protection relays.
• Reverse-power, loss-of-excitation, over/under-voltage and frequency protection are active.
• Earthing arrangement is verified for parallel operation.

Bus and load

• Bus voltage and frequency are within limits.
• Load level and plan for taking load are defined.
• Shedding and dispatch logic are set. See load shedding.

Decision flow

1. Is the bus dead? If yes, use dead-bus closing logic for the first source, with interlocks.
2. If the bus is live, are all four conditions met? If yes, close.
3. If not, adjust speed and voltage, or wait for the synchroniser. Never force a close outside the window.
4. After closing, ramp load on in a controlled way and check sharing.

Commissioning tests

• Phase-rotation test and verification of CT and VT polarity.
• Sync check with the breaker racked out, observing the sync scope.
• Deliberate out-of-window close attempt to confirm the permissive blocks it.
• Reverse-power and load-sharing tests.

See synchronising and paralleling switchgear.

Worked illustration

After a generator overhaul, the alternator leads are reconnected. The unit looks healthy, but phase rotation was changed. At the first synchronising attempt, the check-sync relay prevents closing because the rotation is wrong. This is exactly the failure the readiness check should catch, before an out-of-phase closing breaks a coupling. The fix is to swap two leads and repeat the check.

Readiness checklist before closing

Decision tree

1. Bus dead: allow one set to close after interlock check.
2. Bus live: run automatic synchroniser, supervised by a sync-check relay.
3. Any check fails: do not force; find and fix the cause.

Closing angle illustration

With 0.1 Hz slip and a breaker closing time of 80 ms, the close command issues about 2.9 degrees before zero phase angle. If the closing time is wrongly entered as 200 ms, the contacts touch about 7 degrees off.

Commissioning tests

Verify each sync-check window by injecting out-of-limit conditions and confirming the breaker will not close.

See synchronisation and dead-bus and sync-check logic.

Common mistakes

• Skipping phase-rotation verification after work on connections.
• Bypassing check-sync for convenience.
• Relying on manual synchronising with no check relay.

What to send us

Send your synchronising scheme and controller details. Send your application details. See also DG synchronising panels.

Questions engineers ask

Tolerances are illustrative. Use the manufacturer’s values for your equipment.