In short. A parallel generator plant fails as a plant even when every set is healthy on its own. Service covers three separate layers: the synchronising function that matches voltage, frequency, phase angle and phase sequence before a breaker closes, the load-sharing function that divides kW and kVAr between sets afterwards, and the protection and switchgear that disconnect a set safely when something goes wrong. Each layer is tested on its own terms against the approved settings.
Three functions that are easily confused
When a paralleled plant misbehaves, the complaint arrives as “the generators will not come together” or “one set keeps tripping”. Before anyone opens a panel, the fault has to be placed in the right layer, because the method and the people qualified to touch the settings differ for each.
| Layer | What it does | Typical symptom when faulty | Where to look first |
|---|---|---|---|
| Synchronising | Matches the incoming set to the bus and closes the breaker inside the permitted window | Set runs for minutes without closing; breaker closes with a bang; sync-fail alarm | Voltage and speed bias to governor and AVR, sync-check relay settings, breaker close time |
| Load sharing | Divides real power between governors and reactive power between AVRs | One set carries most of the load; kVAr circulates; reverse power trips at light load | Load-share lines, droop and compensation settings, CT polarity |
| Protection and switchgear | Isolates a faulty set or a faulted bus without taking down the healthy sets | Nuisance trips; a fault takes out the whole plant; breaker fails to open | Relay settings and coordination, trip circuit, breaker mechanism and DC supply |
The pages on synchronisation and load-sharing controls cover the theory. Service is about proving that the installed equipment still does what those pages describe.
Servicing the synchronising function
Before a set can close onto a live bus, four conditions must be inside the window set during commissioning: voltage typically within a few percent, frequency within a fraction of a hertz with a small positive slip so the incoming set takes load rather than motoring, phase angle within roughly 10 degrees allowing for breaker closing time, and the same phase sequence. A set that hunts around the window and never closes usually has a problem in the bias path from the synchroniser to the governor or AVR: a loose wire, a sluggish governor, or a bias gain set too high after a controller replacement. A set that closes with a heavy thump is closing outside the window, because the sync-check relay has been widened or because the breaker’s closing time has grown with wear and no longer matches the controller’s advance angle.
Service therefore measures the breaker closing time, checks the controller’s phase-angle indication against an independent measurement, confirms phase sequence on both sides of the breaker, exercises the governor and AVR bias inputs through their full range with the set off the bus, and injects the sync-check relay to prove it blocks closure outside the window.
Servicing load sharing
Once two sets are on the bus, the governors decide who carries the kW and the AVRs decide who carries the kVAr. Unequal real-power sharing shows as one set running hot and the other near reverse power; unequal reactive sharing shows as circulating current, one AVR at high excitation and the other low, with no change in bus voltage. Both need kW and kVAr readings on every set at the same moment.
The usual causes are a load-share line with a broken screen or a loose terminal, droop settings that differ between sets after one controller or AVR was replaced, a current transformer wired with reversed polarity so the controller sees the load backwards, and governors with different response times, which produce a slow oscillation of load between sets. Service checks each in turn and finishes with a recorded load step, because sharing that looks balanced at steady state can still oscillate during a step. The kVAr sharing calculator helps interpret the readings.
Protection: the layer nobody should adjust casually
Generator protection in a parallel plant exists to disconnect a set that has become a hazard to the others. Reverse power stops a fuel-starved engine being motored by the bus; loss of excitation stops a set with a failed field drawing heavy reactive current; overcurrent, earth fault, voltage and frequency functions protect against faults and a runaway set. When one of these trips repeatedly, the question is why the condition is arising, not which setting can be widened so the set stays on. A reverse-power trip at light load is usually a load-sharing fault; widening the reverse-power setting hides it and lets the next genuine fuel failure motor the engine.
Service verifies relay settings against the approved setting sheet, injects each function to prove pick-up and timing, and tests the trip circuit from relay contact to breaker trip coil including the DC supply and its supervision. Settings are changed only against a documented engineering basis and are recorded. The guides on reverse power and generator protection and synchronising and paralleling switchgear explain the functions in detail.
Breakers and switchgear
The synchronising controls can be perfect while the switchgear quietly deteriorates. Air circuit breakers that close every day and trip under fault once in ten years need their mechanism exercised, contacts inspected for erosion, springs and charging motors checked, and trip coils and auxiliary contacts proven. Busbar joints and terminations are thermally imaged under load and torque-checked at shutdown, and bus insulation resistance is recorded. The DC trip supply is tested under load, because a breaker with a dead trip battery closes happily and never opens. The page on DC control power and trip circuits covers that supply.
Safety and the approved documents
Paralleling switchgear can be energised from several directions, so work is done only under a site-specific isolation and permit procedure by people authorised for that switchgear. Nothing published online substitutes for the plant’s single-line diagram, approved setting sheet and commissioning records; if those cannot be found, recovering them from the installed relays and controllers is the first task. Where the site also synchronises with the DISCOM supply, the utility’s protection requirements apply as well.
Common mistakes
- Widening the sync-check window to stop a sync-fail alarm, so the breaker closes out of phase and the resulting torque shock damages couplings and windings.
- Raising the reverse-power setting to stop trips at light load, which hides a load-sharing fault and removes the protection that stops a fuel-starved engine being motored.
- Replacing one controller or AVR with different droop settings from the rest of the plant, so load sharing becomes unequal and one set runs continuously overloaded.
- Servicing the generators on an AMC while never exercising the breakers or testing the trip battery, so the plant cannot clear a fault when one finally occurs.
What to send us
Send the single-line diagram, the make and model of the generator controllers and the protection relays, the breaker ratings, and a description of the fault sequence with any event logs from the controllers. If you have the commissioning setting sheet, include it; if you do not, say so, because recovering the settings becomes part of the scope. Use the service and AMC enquiry form or, for a plant expansion or re-engineering question, the application engineering enquiry.
Questions engineers ask
Why does one generator keep tripping on reverse power at low load?
At light plant load the governors are sharing a small total and any mismatch in droop or in the load-share signal pushes one set below zero kW. Correct the load sharing and consider a load-dependent start and stop scheme that takes sets off the bus when the load is low, rather than widening the reverse-power setting.
Can generators of different makes and sizes be paralleled?
Yes, provided the controllers can share load on a common basis and the droop or isochronous settings are matched. Different sizes share in proportion to rating when set up correctly. Alternator pitch and reactance differences matter for kVAr sharing, so the AVRs need compensation settings chosen for the pair rather than copied from one.
How often should paralleling switchgear be serviced?
Thermal imaging and a functional check of synchronising and load sharing annually, with secondary injection of protection, breaker servicing and a trip-circuit test every two to three years or as the breaker manufacturer specifies. A plant that is only exercised on no load should also have a loaded parallel run at least once a year.
What happens if a breaker closes out of synchronism?
The sets are forced into step by a current surge that can be several times rated current, producing a torque shock on the coupling and shaft and a mechanical thump heard across the plant. Repeated events loosen windings and crack couplings. Any plant that has had such an event should have the sync-check relay, breaker closing time and controller advance angle checked before it is paralleled again.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators. Last updated 19 October 2026.