In short. Synchronisation is closing a breaker between a running generator and a live bus at the instant when phase sequence, voltage, frequency and phase angle match closely enough that no damaging current flows. Typical closing windows are 5 percent on voltage, 0.1 to 0.2 Hz on frequency and 10 electrical degrees on angle. An automatic synchroniser adjusts the incoming set and issues the close; a sync-check relay should supervise it. A close outside the window can shear couplings and damage windings.
Why the four conditions matter
Two alternating sources can be joined only when their voltage waveforms line up. Phase sequence must be the same, or two phases are effectively crossed and the close is a short circuit. Voltage magnitude must match, or reactive current circulates between the machines in proportion to the difference. Frequency must match, or the phase angle is changing continuously and the contacts close at an angle the synchroniser did not intend. Phase angle must be near zero, because the torque on the incoming rotor at closing rises with the sine of the angle difference, and a close at 30 degrees or more delivers a shock that the coupling, crankshaft and stator wedges were not designed to take. Phase sequence is checked once at commissioning with a rotation meter on both sides of the breaker; the other three are checked at every closure.
Closing windows
| Quantity | Typical permissive window | Why the limit is there | Who adjusts it |
|---|---|---|---|
| Voltage difference | Within 3 to 5 percent of nominal | Limits reactive current surge at closing | AVR via voltage matching signal |
| Frequency difference (slip) | 0.1 to 0.2 Hz, incoming set slightly fast | Keeps angle change slow enough for breaker timing; set takes load rather than motoring | Governor via speed bias signal |
| Phase angle | Within 5 to 10 electrical degrees at contact touch | Limits torque shock and current surge | Synchroniser timing, compensated for breaker close time |
| Phase sequence | Identical | Any mismatch is a fault | Fixed by cabling, verified at commissioning |
These are typical settings for low-voltage diesel sets from 100 kVA to 2 MVA and should be confirmed against controller and alternator manufacturers’ guidance; tighter windows apply to large machines and grid paralleling. The incoming set is run marginally faster than the bus so that it picks up a little real power at closing instead of motoring, which would trigger reverse-power protection within seconds.
Breaker close time and the angle at contact
Angle at closing = Angle at command + 360 × Δf × tc
Here Δf is the slip frequency in hertz and tc is the breaker closing time in seconds, so the result is in electrical degrees. The synchroniser must issue its close command ahead of the zero-angle point by exactly this amount, which is why the breaker’s closing time is a setting in every automatic synchroniser. For an ACB closing in 60 milliseconds with a slip of 0.15 Hz (illustrative), the angle moves 3.2 degrees between command and contact touch, so the command must be given at about 3 degrees before coincidence. If the breaker’s mechanism slows with age or lubrication, the actual closing angle drifts, and this is the usual reason a synchroniser that worked at commissioning begins producing rough closures years later. The automatic synchronising control architecture page describes how the compensation is set and checked.
Methods of synchronising
Automatic synchronising
The generator controller or a dedicated synchroniser measures both sides of the breaker, sends speed bias to the governor and voltage bias to the AVR to bring the incoming set into the window, and issues the close command when the predicted angle at contact is within limit. This is the standard for every paralleled diesel plant and for AMF systems with closed transition. It should be backed by an independent sync-check relay, ANSI 25, that blocks the close circuit unless the conditions are met, so that a controller fault cannot close the breaker out of phase.
Manual synchronising with check relay
An operator uses a synchroscope or lamps, adjusts speed and voltage by hand, and presses close when the pointer approaches twelve o’clock. A sync-check relay must still supervise the close. Manual mode is retained on most panels as a fallback and for training, but with modern controllers it is rarely the normal method.
Dead-bus closing
When the bus is de-energised, there is nothing to synchronise with. The first set closes onto the dead bus on a dead-bus permissive, and every subsequent set synchronises to it. The logic must ensure that only one set can receive a dead-bus permissive at a time, otherwise two sets may close onto the same dead bus simultaneously, out of phase with each other. The dead-bus, live-bus and sync-check control logic page explains the interlocking, and the black start guide covers the wider restoration sequence.
What synchronisation does not prove
A clean closure shows that the two machines were aligned at one instant. It says nothing about what happens next. Within a few seconds the governors and AVRs must settle into a stable division of real and reactive power, and the protection must tolerate the transient without tripping. Sites regularly report a successful synchronisation followed by a reverse-power trip or a kVAr oscillation; these are load-sharing faults, not synchronising faults, and are covered on the kW and kVAr load sharing page. Paralleling with the DISCOM supply adds protection and permission requirements of its own, described on the utility paralleling page.
Commissioning checks
- Phase sequence confirmed on both sides of every paralleling breaker with the same instrument, and the VT wiring to the synchroniser verified phase by phase.
- Breaker closing time measured and entered into the synchroniser, not left at a default.
- Sync-check relay tested by attempting a close with a deliberate voltage and angle error and confirming it blocks.
- First live synchronisation witnessed with a recording meter on the generator breaker; a clean close shows only the small load pick-up from the slip.
- Dead-bus logic tested by de-energising the bus and confirming that exactly one set closes first.
Related photographs and videos
Common mistakes
- Relying on the controller’s internal synchroniser with no independent sync-check relay, so a single sensing fault can close the breaker out of phase.
- Leaving the breaker closing time at the factory default, which shifts the true closing angle by several degrees on slow or ageing mechanisms.
- Running the incoming set slower than the bus, so it motors at closing and trips on reverse power before it can take load.
- Assuming that identical controllers on identical sets need no site-specific settings, when VT ratios, breaker types and cable lengths all differ.
What to send us
Send the single-line diagram of the paralleling switchgear, the make and model of the generator controllers and any separate synchroniser or sync-check relay, the breaker type and its stated closing time, and a photograph of the controller settings screen for the synchronising parameters. If closures are rough, a description of what is observed and any event log from the controller is valuable. Use the application engineering enquiry and we will review the settings and the supervision.
Questions engineers ask
What happens if a generator is synchronised out of phase?
A large current flows between the machines, limited only by their reactances, and the incoming rotor is pulled violently towards alignment. At large angles the torque can reach several times rated, damaging the coupling, crankshaft, alternator wedges and breaker. Even if nothing breaks visibly, the alternator and coupling should be inspected before the set returns to service.
How long should synchronising take?
With a healthy governor and AVR an automatic synchroniser normally brings a set into the window and closes within 10 to 30 seconds of reaching rated speed. Times beyond a minute, or repeated failures to close, usually point to a slow governor response, an excessive slip setting or a dead band in the speed bias.
Can two generators of different makes be synchronised together?
Yes, provided each has a governor and AVR that accept bias signals and both are controlled by compatible load-sharing equipment. Synchronising itself is indifferent to the make. The difficulty with mixed plants is in load sharing afterwards, where droop settings and response speeds must be matched.
Is a sync-check relay necessary if the controller already synchronises?
We regard it as essential. The controller both decides and acts, so an error in its sensing or logic closes the breaker wrongly. A separate relay with its own VT inputs that interrupts the close circuit is a low-cost safeguard against the most expensive failure in a paralleling plant.
Why does the set trip on reverse power immediately after closing?
The incoming set was running at or below bus frequency when it closed, so the bus drove it as a motor and the reverse-power relay operated. Set the synchroniser for a small positive slip and check that the governor takes a load step immediately after the close signal.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.