In short. A synchronising panel is the control and protection cubicle that brings a generator onto a live bus safely. It matches speed, voltage and phase, closes the breaker only when conditions are safe, and then manages load sharing and protection.
Panel contents
- Generator controller with synchronising, load sharing and protection functions.
- Sync-check relay acting as an independent permission to close.
- Breaker control with open, close and trip circuits, spring charge and status indication.
- Protection relay (if separate from the controller).
- Metering: voltage, current, kW, kVAr, power factor, frequency.
- Selector switches for auto, manual and test modes.
- Emergency stop and alarm annunciation.
- DC supply and fusing for control.
How synchronising works
- The controller measures voltage and frequency on both sides of the open breaker.
- It adjusts the incoming generator’s speed (and voltage) to match the bus.
- When frequency, voltage and phase angle are within the closing window, the controller issues a close signal.
- The sync-check relay confirms permissive conditions; otherwise the close is blocked.
- After closing, load sharing takes over.
Closing out of phase can cause large currents and torque, damaging the shaft, coupling and windings.
Manual synchronising
Some panels provide manual synchronising with a synchroscope and lamps. It should always be supervised by a sync-check relay, so that an operator error cannot close an out-of-phase breaker.
Commissioning tests
- Phase sequence and phase relationship check across the breaker.
- Voltage and frequency matching, and operation of the closing window.
- Sync-check blocking test with an intentionally wrong condition.
- Load sharing check across the load range.
- Protection and trip tests. See switchgear testing and commissioning.
Illustrative example
During commissioning, a technician swaps two phases at a generator terminal. The phase-sequence check shows a mismatch, and the sync-check relay blocks the breaker. After correcting the connection, synchronising proceeds. Without the independent relay, the first closing would have been out of phase.
What a DG synchronising panel contains
| Section | Function |
|---|---|
| Generator incomer breaker | Connects each set to the common bus |
| Common bus | Distributes power to load feeders |
| Synchronising controller per set | Matches and closes |
| Load-sharing and power management controller | Shares kW and kVAr, starts and stops sets by demand |
| Protection relays | Per generator and for the bus |
| Metering and HMI | Operator visibility |
Design points
- Bus rating must exceed combined fault contribution of all sets plus any utility source.
- Breaker closing time and control voltage must suit the synchroniser.
- Provide a fallback manual sync mode with sync-check.
- Define priority and standby order for sets.
Dead-bus start logic
Define who closes first. A bus-voltage check and an interlock stop two incomers closing onto a dead bus at once.
Control power
Provide reliable DC supply for controllers and breaker coils, with redundancy for critical sites. See DC control power.
Testing and commissioning
- Functional tests of synchronising with each set.
- Load-share tests at stepped loads.
- Fault-case tests of protection and interlocks.
See multi-DG load sharing and commissioning.
Common mistakes
- Skipping phase-sequence checks.
- Relying on the synchroniser without independent sync-check.
- Poor DC supply to the breaker coil.
What to send us
Send the generator and bus details. Send your application details. See also paralleling switchgear.
Questions engineers ask
What is a synchroscope?
An indicator showing the phase difference and relative speed between two sources.
Why is sync-check separate?
So that a single failure in the synchroniser cannot cause an unsafe close.
Can one panel serve several generators?
Usually each generator has its own panel or section, connected by a bus and communication.
How often should it be tested?
As part of regular testing, and after changes to controls or breakers.
This guide gives general principles. Follow the manufacturer’s commissioning procedures.