In short. A synchronising panel is the switchgear and control assembly where independent diesel generators are brought together onto one bus. It houses the generator breakers, the controllers that sense each source and check synchronism, the load-sharing and sequencing logic, protection relays, metering and the operator interface. Its quality is judged by how clearly it shows the plant state and how safely it handles the abnormal cases: dead bus, failed synchronisation, breaker failure and controller loss.
What a synchronising panel integrates
The panel is less a single product than a coordinated set of functions in one or more cubicles. A complete panel for a multi-set LV plant includes the following.
- One incoming breaker per generator, motorised or with a closing release, with auxiliary contacts for position feedback and a shunt trip.
- A common bus, often with a bus coupler on larger plants, and outgoing feeder breakers.
- A generator controller per set for engine control, sensing, automatic synchronising, load sharing and set-level protection.
- A master controller, where there are more than two or three sets, holding sequencing, priority, load-dependent start and stop, and load management.
- Protection relays independent of the controllers: synchronism check on each breaker, reverse power, voltage and frequency, earth fault and overcurrent.
- Metering per generator and for the bus, a mimic or HMI, mode selectors, emergency stop, and the gateway to SCADA or BMS.
- Control power from a DC system or diode-combined generator batteries, so the panel works with the bus dead.
Controller functions are described under synchronising controls and the breakers and bus under LV generator switchgear; what follows is how they come together.
Core sequences the panel must execute
Dead-bus start
After a complete loss of power, all sets receive a start command. The panel must allow exactly one set to close onto the de-energised bus without synchronising and hold the others until the bus is live. Controllers normally arbitrate over the communication link, with a hard-wired interlock as backup so that a link failure cannot let two breakers close at once. The wider procedure is under black start.
Synchronising to a live bus
Each set’s controller matches voltage, frequency and phase to the bus and issues a close through the synchronism-check relay. The acceptance window, the maximum time allowed before an alarm, and the behaviour on failure must all be defined.
Load-dependent start and stop
The master controller starts the next set in priority order when load exceeds roughly 80 to 90 percent of online capacity, and stops a set when load has been low enough for long enough that the remaining sets keep reserve. Hysteresis and delay prevent cycling, and priority can rotate on running hours.
Unloading and opening
A set being stopped is ramped down to a few percent of load before its breaker opens, so the others pick up smoothly and the departing set does not see reverse power. Cooldown and stop follow.
Panel types and their engineering boundaries
| Type | Sources combined | Requirements beyond a single-set AMF panel |
|---|---|---|
| Generator-to-generator (two sets) | Two diesel sets on one bus | Synchronising, kW and kVAr sharing, reverse power, dead-bus logic |
| Multi-generator | Three or more sets, possibly with bus sections | Master controller, load-dependent start and stop, bus coupler logic, higher fault level, load management |
| Generator-to-utility | One or more sets with the DISCOM supply | Interface protection, loss-of-mains detection, import or export control, DISCOM approval |
The utility case is a separate regulatory project, covered under generator-grid paralleling. A panel built for generator-to-generator operation cannot simply be switched to utility paralleling later without the interface protection and approvals being added.
The operator interface
A synchronising plant is operated by people under pressure at odd hours, and the panel front is their only view. Good design answers five questions at a glance: which sources are available, which breakers are closed, what alarms are active and which are new, how much load each set and the bus carry, and what mode the plant is in. A mimic with illuminated breaker symbols or an HMI with the same single-line view does this; a row of identical controllers with small screens does not. Manual synchronising facilities, a synchroscope or check-sync lamps and a close pushbutton supervised by the sync-check relay, should be provided so the plant can be run if the automatic function is lost.
Specifying a panel
A usable specification states the number and rating of sets now and in future, bus and breaker ratings including the fault level with all sets online, the control philosophy in words (dead-bus logic, synchronising windows, sharing mode, start and stop thresholds, priority and rotation), every protection function and who sets it, the interface list to SCADA or BMS, the control power arrangement, the enclosure rating, and the commissioning tests to be witnessed. The make of controller is secondary once these are defined.
Commissioning
Commissioning is structured testing of each layer before the next is energised. Control wiring and interlocks are checked with the breakers racked out. Each set is run alone and its sensing, metering and phase sequence confirmed at the panel terminals. The sync-check relay is tested by injection and the automatic synchroniser proven with the close coil disconnected. The first live synchronisation is at light load with a recorder on current, and load sharing is tested with a load bank in steps. Then the abnormal cases: dead-bus start with all sets ready, loss of the communication link, breaker failure to close and to open, a set tripped on load, loss of control power, and master controller shutdown with the plant running. Our commissioning and synchronising and switchgear service teams run this sequence and the record becomes the maintenance baseline.
Retrofitting and modernising older panels
Many plants in Hyderabad’s older industrial areas have panels from the 1990s or 2000s with electromechanical relays, analogue load-sharing modules and sets from more than one manufacturer. Modernising them with digital controllers and an HMI is usually worthwhile, but each set’s governor and AVR must be able to accept the new controller’s bias signals; a mechanically governed engine will need upgrading or will have to run in droop. The retrofit should start with a survey of each machine’s interfaces and a review of protection and sequence, and end with the full commissioning sequence above. See legacy switchgear modernisation.
Common mistakes
- Relying on the communication link alone for dead-bus arbitration; a link fault lets two sets close onto a dead bus together and they connect out of phase.
- Taking control power from the bus; when the bus is dead the panel cannot start the first set or close its breaker.
- Omitting an independent sync-check relay because the controller checks synchronism; a controller fault or wrong wiring then closes a breaker out of phase with nothing to stop it.
- Accepting the panel after a demonstration of normal synchronising only; the first real event is a breaker failure or a controller loss that was never tested.
What to send us
Send the nameplates and controller, governor and AVR types of every set you want on the panel, the load profile or load list, a single-line diagram or sketch of the intended bus and feeders, photos of the existing panel front and inside if it is a retrofit, and the space and ventilation available for the panel. Tell us whether utility paralleling is wanted now or in future, because it changes the design. We will propose the panel architecture, protection and commissioning plan. Use the application engineering enquiry.
Questions engineers ask
What does a synchronising panel do?
It senses each generator and the bus, brings each set into synchronism before closing its breaker, shares kW and kVAr between the connected sets, starts and stops sets as load changes, protects against reverse power and faults, and gives the operator a clear view of the plant state and alarms. It is where separate machines become one coordinated supply.
What is the difference between an AMF panel and a synchronising panel?
An AMF panel manages one generator against the mains: it detects failure, starts the set and transfers the load with a break. A synchronising panel manages several generators together on a common bus, with synchronising, load sharing and sequencing. Many plants have both, with the synchronising bus feeding the AMF changeover.
Can old generators be connected to a new synchronising panel?
Usually, if their governors and AVRs can accept external bias signals or can be upgraded to ones that do. Mechanically governed sets can sometimes run in droop alongside isochronous sets under a master controller, but with less precise sharing. A survey of each machine’s interfaces decides it.
How is the first generator connected when all power is lost?
Through dead-bus logic. The controllers decide which ready set closes first onto the de-energised bus without synchronising, usually the highest-priority set or the first to reach rated voltage, and a hard-wired interlock ensures only one breaker can close while the bus is dead. The rest then synchronise to the live bus.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.