In short. Paralleling switchgear connects two or more generators to a common bus. It must bring them into synchronism before closing, share load and VAr between them, protect the bus and each generator, and manage start, stop and failure of units.
Main parts
| Part | Function |
|---|---|
| Generator breakers | Connect each generator to the bus |
| Bus | Rated for combined fault level |
| Synchroniser and sync-check | Match voltage, frequency, phase before closing |
| Load-share controller | Splits kW and kVAr between sets. See load sharing |
| Master controller | Starts and stops units by demand, handles faults |
| Protection relays | Per generator and bus |
| Tie / coupler and utility breakers | Connect to the rest of the system |
Synchronising conditions
Before closing, the incoming generator must match the bus in voltage, frequency and phase angle within narrow limits. Automatic synchronisers adjust speed and voltage; a sync-check relay independently blocks closing if conditions are not met. See synchronisation.
Fault level of the bus
With several generators, the fault level adds up, and the bus and breakers must be rated for the total. The fault current of all sets plus any utility contribution defines the rating. See fault levels.
Specification checklist
- Number of generators now and in the future, and spare breaker positions.
- Bus rating and short-time withstand.
- Synchronising method: automatic with sync-check backup.
- Load sharing mode: isochronous, droop, or communication-based.
- Dead-bus closing logic with lockouts so that two sets do not close on a dead bus together.
- Protection per generator and bus.
- Control power redundancy. See DC and trip circuits.
- Factory test with live generators or simulators.
Illustrative example
A three-generator plant with 1,000 kVA sets has a 2,000 A bus. The first set to start closes onto the dead bus, the others synchronise and close, and load is shared proportionally. If one set trips, the master controller starts a standby unit and sheds non-essential load if capacity falls short.
The four conditions before closing
Paralleling two AC sources needs matched voltage, frequency, phase angle and phase sequence at the instant of closure. Mismatch creates a surge of current and torque on the machines.
Approximate surge current ≈ ΔV ÷ (Xd” + Xsystem), where ΔV is the vector voltage difference at closing
Typical synchronising windows (illustrative)
| Parameter | Typical window |
|---|---|
| Voltage difference | Within about 5 to 10 percent |
| Frequency difference | Within about 0.1 to 0.2 Hz |
| Phase angle | Within about 10 to 20 degrees at closing |
Confirm values from the relay maker and the machine data.
Equipment in a paralleling board
- Synchroniser or sync-check relay.
- Governor and AVR control interfaces.
- Load-sharing controller.
- Generator breaker with suitable closing time.
- Protection relay package.
Auto vs manual
Automatic synchronisers are standard on modern sets. Manual synchronising with a synchroscope should be backed by a sync-check relay to prevent a bad close.
Dead-bus and live-bus closing
When the bus is dead, the first generator closes onto it. Logic must stop two sets closing onto a dead bus simultaneously.
Related topics
See synchronisation and kW and kVAr load sharing.
Common mistakes
- Bus rated for one generator, not all.
- No sync-check independent of the synchroniser.
- Dead-bus closing without arbitration.
What to send us
Send the generator list, load profile and single-line diagram. Send your application details. See also synchronising switchgear service.
Questions engineers ask
Can different generator sizes run in parallel?
Yes, with proportional load sharing, as long as controls are compatible.
What if sync-check fails?
Closing is blocked. Check speed, voltage and phase sequence.
What is a dead-bus closing?
The first unit closing on an unenergised bus, which needs logic to avoid two closing together.
Is a master controller necessary?
For plants with several units, it manages start, stop, sharing and shedding.
This guide gives general principles. Specification needs project data.