In short. Synchronising connects generators to a bus; load sharing decides how much each one carries afterwards. Real power (kW) is divided by the engine governors, reactive power (kVAr) by the alternator AVRs, and the two are set up and tested separately. Sharing can be by droop, where each set’s frequency and voltage fall slightly with load, or isochronous, where controllers exchange signals to hold frequency constant while balancing load. Unequal sets should share in proportion to rating, not equally.
Two loads, two control systems
A three-phase load draws real power, which does work and comes from the engine’s fuel, and reactive power, which magnetises motors and transformers and comes from the alternator’s excitation. On a common bus each has to be divided among the machines, and the dividing is done by different hardware.
- kW sharing is governed by the engines. If one governor delivers slightly more fuel than its share, that engine pushes more kW into the bus and the others back off. Taken to the extreme, one engine carries everything and the others are driven as motors.
- kVAr sharing is governed by the AVRs. If one AVR holds a slightly higher voltage setpoint, that alternator supplies more reactive current, and current circulates between the machines without doing useful work.
A plant whose kW is balanced can still have one alternator at 90 percent of rated current and another at 40 percent because of kVAr imbalance. Both loops must be engineered and measured at commissioning.
Droop sharing
Droop is the simplest method. Each governor lets engine speed, and therefore bus frequency, fall by a defined percentage from no load to full load, typically 3 to 5 percent. Each AVR likewise lets voltage fall by 3 to 5 percent from no load to full reactive load, using a droop CT on one phase. Because all sets see the same bus frequency and voltage, each settles at the load that corresponds to that frequency on its own droop line, with no wiring between controllers. That is why droop remains the fallback mode in every modern plant and the only mode available on mechanically governed sets.
The price is that frequency and voltage vary with load and must be trimmed by an operator or master controller. For a standby plant running a few hours a month this is acceptable; for a prime plant feeding frequency-sensitive loads it is not.
Isochronous load sharing
In isochronous mode each governor holds speed exactly at setpoint regardless of load. Two such governors on one bus would fight, so the controllers are connected by a load-sharing line over which each announces its percentage load, and each biases its own governor until all sets are equal while the bus stays at 50.0 Hz. The same applies to AVRs for kVAr, through cross-current compensation or a digital reactive sharing signal. Isochronous sharing is standard on new plants and depends on the integrity of the sharing line, so good designs fall back to droop automatically if the line is lost. Controller-level detail is under load-sharing controls.
Proportional sharing between unequal sets
Where sets have different ratings, equal kW sharing would overload the smaller set first. The correct target is that every set runs at the same percentage of its own rating.
P_i = P_total × S_i / (S_1 + S_2 + … + S_n)
Here P_i is the kW set i should carry, P_total is the total bus load in kW, S_i is the rating of set i, and the denominator is the sum of the ratings of all sets online. Worked example (illustrative): a 500 kVA set and a 750 kVA set share 800 kW. The 500 kVA set should carry 800 × 500 / 1,250 = 320 kW and the 750 kVA set 480 kW, so both run at 64 percent of rated kW at 0.8 power factor. Equal sharing at 400 kW each would put the smaller set at 100 percent while the larger ran at only 67 percent. Modern controllers do this automatically when told each set’s rating; droop systems achieve it with the same percentage droop on each machine.
What poor load sharing looks like
| Symptom | Likely cause | Where to look |
|---|---|---|
| One set at high kW, another near zero or negative | Governor setpoint mismatch, sharing line fault, mixed droop and isochronous modes | Controller mode settings, sharing line wiring and screen |
| Reverse-power trip shortly after paralleling | Engine not taking load fast enough, fuel problem, governor gain too low | Fuel filters, actuator, governor ramp settings |
| Equal kW but very different alternator currents | kVAr imbalance from AVR setpoint or droop CT polarity | AVR voltage trim, droop CT connection |
| Bus voltage oscillating slowly | AVR gains too high for parallel operation | AVR stability setting, compensation wiring |
| Alternator overheating at moderate bus load | Circulating reactive current between machines | kVAr per set, winding pitch mismatch |
Reverse power is the protection most often blamed on the relay. A set in parallel that stops producing torque is immediately driven as a motor by the others, and the reverse-power relay, typically set at 5 to 15 percent of rating with a delay of a few seconds, trips it to protect the engine. The relay is doing its job; the cause is in the engine or the sharing. See reverse power and generator protection.
Load sharing within the wider operating strategy
A plant controller that starts and stops sets to follow demand must coordinate with the sharing so that a newly synchronised set ramps up gently rather than stepping to its share, and a set being stopped is unloaded before its breaker opens. It must also hold a minimum online reserve for sudden steps such as a chiller start. These sequencing functions sit in the master controls, and what to shed if the reserve is lost sits in load management.
Setting up and proving load sharing
A workable commissioning sequence runs as follows. Verify each set alone for stable voltage and frequency and correct droop CT polarity. Parallel two sets at light load and watch kW and kVAr per set; they should reach their targets within 10 to 20 seconds. Apply load in steps with a load bank and repeat, noting any oscillation. Disconnect the sharing line deliberately and confirm the plant falls back to droop without tripping. Finally trip one set on load and confirm the others pick up its share without an excessive frequency excursion. Record every reading as the baseline for future service. Our load-bank testing service provides the controlled load for these steps.
Related photographs and videos
Common mistakes
- Commissioning kW sharing and never looking at kVAr per set; one alternator runs hot carrying the plant’s reactive load alone.
- Setting equal sharing between sets of different ratings; the smaller set reaches 100 percent while the larger is half loaded.
- Disabling reverse-power protection because it “keeps tripping”; the next time an engine loses fuel it is motored until it is damaged.
What to send us
Send the nameplates and controller types of each set, the governor and AVR types if the sets are older, photos of the controller screens showing kW, kVAr and current per set while running in parallel, any trip or alarm records, and a note of the symptom: uneven load, reverse-power trips, hunting or overheating. If the plant is new, send the single-line diagram and the intended number of sets. We will diagnose the sharing problem or specify the control scheme. Use the service and AMC enquiry for an existing plant.
Questions engineers ask
What is the difference between synchronising and load sharing?
Synchronising is the act of matching a generator’s voltage, frequency and phase to a live bus so that its breaker can close safely. Load sharing is what happens afterwards: the continuous control that decides how much kW and kVAr each connected set carries. A plant can synchronise perfectly and share badly.
Should I use droop or isochronous sharing?
Isochronous sharing is preferred for any plant that runs for extended periods or feeds frequency-sensitive loads, because the bus stays at 50 Hz. Droop is simpler, needs no interconnection, and is the right fallback and the right choice for older sets or very short parallel periods. Most modern plants run isochronous with automatic fallback to droop.
What causes reverse power in a paralleled generator?
Reverse power occurs when a set stops producing mechanical power and is driven as a motor by the bus. Common causes are fuel starvation, a sticking actuator, a governor that unloads too quickly, or a breaker that closed before the engine was ready to take load. The relay trips the set; the fix is in the engine or the control settings.
Why does one generator take all the load?
Almost always because its governor setpoint or droop setting differs from the others, or because the load-sharing line between controllers is broken or wrongly wired. Check that all sets are in the same sharing mode and that each controller displays the others’ load; if it does not, the sharing line is the problem.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.