In short. Closing a generator onto a live bus out of step can shear couplings, twist shafts, damage windings and wreck breakers in a fraction of a second. Before any set synchronises, four electrical conditions must match: voltage, frequency, phase angle and phase sequence. The breaker, controls and protection must also be ready. This checker walks through the readiness questions in order, so the first close is made only when every answer is yes.
Who this page is for: For commissioning engineers, operators and service teams synchronising generators to each other or to the grid.
Key facts
- The four conditions: voltage, frequency (slip), phase angle and phase sequence.
- Typical sync-check windows: voltage within a few percent, slip a fraction of a hertz, angle within about 10 degrees.
- Phase sequence is proved once at commissioning, and again after any cable or VT work.
- An independent sync-check relay supervises the synchroniser.
- The incoming set should run slightly fast so it picks up load, not motors.
- Manual closing is the riskiest method; use automatic synchronising with sync-check supervision.
The four synchronising conditions
| Condition | Typical window (illustrative) | If exceeded |
|---|---|---|
| Voltage difference | Within a few percent | Reactive current surge at closing |
| Frequency difference (slip) | A fraction of a hertz, incoming set slightly fast | Power surge; reverse power if the set is slow |
| Phase angle | Within about 10 degrees at the moment of closing | Torque shock and high transient current |
| Phase sequence | Identical rotation | Severe fault on the first close |
Readiness checklist
| Area | Ready when |
|---|---|
| Engine | Stable at rated speed, no active alarms, warm enough for load |
| Alternator | Voltage stable, regulator in auto, droop or sharing mode correct |
| Breaker | Spring charged, close and trip circuits healthy, DC supply good |
| Phase rotation | Proved and recorded for all sources |
| Synchroniser and sync-check | Healthy, correct windows and breaker closing-time compensation |
| Dead-bus and live-bus logic | Correct; VT supervision active. See dead-bus and sync-check logic |
| Protection | Generator protection enabled, reverse power and overcurrent set for parallel operation |
| Load sharing | Governors and regulators in the intended mode, link healthy |
Worked example: reading a bad close from the record (illustrative)
After a new set tripped on reverse power at its first parallel, the controller’s record showed:
| Quantity at closing | Recorded | Finding |
|---|---|---|
| Voltage difference | 2 percent | Inside the window |
| Slip | Incoming set 0.15 Hz slow | Set closed slow, so it motored and tripped on reverse power |
| Phase angle at the breaker contacts | About 13 degrees | Outside the window: the close command was given at 8 degrees, but the breaker’s closing time of about 90 ms was not compensated |
Two fixes followed: the synchroniser was set to bring the set in slightly fast, and the breaker’s closing time was entered so the command is issued early enough for the contacts to meet near zero degrees. The sync-check relay was then confirmed to block any close command outside its 10-degree window. See synchronising and paralleling switchgear.
Never bypass a sync-check relay to get a set on the bus. If it blocks a close, find out why. An out-of-phase close can do more damage in one cycle than years of normal running.
Synchronising methods
- Automatic synchroniser with sync-check: consistent and the preferred method.
- Manual with sync-check supervision: acceptable with trained staff.
- Manual without supervision: should not be used.
Synchronising readiness sheet
| Check | Result | Initials |
|---|---|---|
| Phase rotation proved | ||
| Sync-check windows and closing-time compensation set | ||
| Breaker and DC healthy | ||
| Protection enabled for parallel operation | ||
| Load-sharing mode correct |
Common mistakes
- No breaker closing-time compensation.
- Bringing the set in slow, so it motors.
- Phase rotation not re-checked after cable work.
- Bypassing the sync-check relay.
What to send us
Send the controller or synchroniser model, settings, and any records from a failed parallel. WhatsApp 98852 01608 or use the enquiry form.
Evidence note. Windows are typical values for illustration; makers’ settings and the utility’s requirements govern.
Questions engineers ask
What happens if a generator closes out of phase?
It can cause severe mechanical shock and high currents, damaging couplings, windings and breakers.
Is automatic synchronising safer than manual?
Automatic synchronisers with check-sync supervision are generally more consistent. Manual synchronising should always have a check relay.
How do I verify phase rotation?
With a phase-rotation meter on each source, or by sync-check measurement before energising.
Can I synchronise to a dead bus?
You do not synchronise; you close onto it under dead-bus logic with interlocks.
Tolerances are illustrative. Use the manufacturer’s values for your equipment.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators. Last updated 8 November 2026.