In short. Paralleling two sources is safe only when voltage, frequency, phase angle and sequence match at the instant of closing. After closing, sets must share kW and kVAr correctly. The study defines the matching windows, the controls, the protection and what happens if any part fails.
Closing conditions
| Condition | Why it matters |
|---|---|
| Phase sequence | Wrong sequence causes a short circuit |
| Voltage | Difference drives reactive circulating current |
| Frequency | Difference makes the angle drift; slip should be small |
| Phase angle | Large angle at closing gives a mechanical and electrical shock |
Controllers use a check-sync window on voltage, slip and angle, with a breaker closing time compensation so that contacts meet at zero angle.
After closing
- kW sharing follows the governor: droop or isochronous with load sharing lines. See load sharing.
- kVAr sharing follows the AVR: droop or cross-current compensation.
- Circulating currents arise from voltage differences, winding pitch and third harmonics.
- Reverse power protection trips a set that begins to motor. See reverse power protection.
Study steps
- List the sources and the busbar arrangement.
- Define sync windows from set and breaker data. See synchronisation.
- Check breaker closing and withstand ratings for the paralleled fault level.
- Calculate expected sharing and verify governor and AVR settings.
- Set protection: reverse power, loss of excitation, over-current, sync-check relay.
- Define failure responses: failed sync, bus dead, set trip, controller loss.
- Commission with a witnessed test sequence. See synchronising switchgear.
Dead-bus and utility parallel
For dead-bus closing, interlocks must prevent two sets closing on the dead bus at once. For utility parallel, the utility may impose protection and export rules. See interconnection study.
Illustrative example
Two 1,000 kVA sets share a bus. On test, one set takes 70 percent of the kW because its governor droop was set differently. The study confirms the cause, standardises the droop and verifies sharing within a few percent across the load range.
Scope of a paralleling study
Before generators share a bus, the study confirms they can synchronise, share load stably and survive a fault together.
| Topic | Check |
|---|---|
| Machine compatibility | Matching voltage, frequency, phase sequence, governor and AVR types |
| Pitch of windings | Different winding pitches in paralleled alternators influence third-harmonic circulating current |
| Fault level | All sets plus utility define bus rating |
| Load sharing | kW through governors, kVAr through AVR droop or cross-current compensation |
| Protection | Reverse power, loss of excitation, differential |
| Dead-bus logic | Who closes first |
Circulating currents
Unequal excitation causes reactive current to circulate between machines. Unequal winding pitch causes third-harmonic circulating current through the neutral. Both are avoided by matched machines and, where needed, by neutral treatment.
Stability
Machines must hold synchronism through a disturbance such as a fault or large load step. Governor hunting, mismatched droop and weak ties can cause oscillation. Transient simulation tests the response.
Utility paralleling
If paralleling with utility is intended, add interconnection study and utility approval. See utility-DG interconnection.
See synchronisation and synchronising switchgear.
Common mistakes
- Check-sync window too wide.
- Different governor or AVR settings across sets.
- No dead-bus interlock.
What to send us
Send set data and the paralleling controller. Send your application details.
Questions engineers ask
What happens if closed out of phase?
High current and torque can damage the alternator, coupling and breaker.
Can different sizes be paralleled?
Yes, with proper sharing settings and protection.
What is droop?
A characteristic where frequency or voltage falls slightly with load so sets share it.
Do I need synch relays and a controller?
Modern controllers include both, but an independent check-sync relay adds safety.
This guide gives general principles. Follow manufacturer instructions and safety practice.