In short. Real power (kW) sharing between parallel generators is controlled by their engine governors. In droop mode each unit’s frequency falls slightly with load, which lets them share naturally. In isochronous mode a load-sharing signal keeps the frequency constant while the units share. Poor sharing shows up as one unit carrying more than its share.
Why sharing matters
Unequal sharing overloads one engine while others run light, risks reverse power and trips, and wastes fuel. Good sharing keeps every running unit at a similar percentage of its rating.
Droop sharing
Each governor is set so frequency falls with load by a droop percentage, such as 3 to 5 per cent from no load to full load. Units on the same bus see the same frequency, so each takes the load that matches that frequency on its own droop line. Equal droop settings give proportional sharing.
Example: 51.5 Hz at no load and 50 Hz at full load gives 3 per cent droop on a 50 Hz system. The system frequency then falls as load rises, so a speed-trim function or a secondary controller restores it.
Isochronous sharing
With isochronous governors, each unit tries to hold frequency, so units would fight each other. A load-sharing line between controllers tells each unit its share and lets one or all hold the frequency without conflict. Alternatively, one unit is isochronous and the others run droop.
Comparison
| Mode | Frequency | Sharing method | Notes |
|---|---|---|---|
| Droop | Falls with load | Natural, from droop lines | Simple; frequency drift needs correction |
| Isochronous with load-share line | Constant | Controller communication | Standard in modern plants |
| One isochronous, others droop | Constant | The swing unit takes the balance | Swing unit sees all load changes |
Typical causes of poor sharing
- Different droop settings or governor calibration.
- Faulty or disconnected load-sharing line.
- Engine or governor differences, such as a sticky actuator.
- Mixed unit sizes without proportional settings.
- Mismatched speed references.
- Measurement error in the power transducers.
Checks
- Compare each unit’s kW as a percentage of rating at several loads.
- Verify droop and gain settings are identical (for equal units) or proportional.
- Check the load-sharing line wiring and shielding.
- Check for reverse power and its relay settings. See reverse power and generator protection.
- Test sharing during steps and during a unit trip.
Worked example (illustrative)
Two 800 kW units serve a 1,000 kW load. Perfect sharing is 500 kW each (62.5 per cent). Measurement shows 600 and 400 kW. A 5 per cent mismatch in droop settings is found and corrected, and sharing returns to within 2 per cent. In practice acceptable tolerance is set by the manufacturer or specification, often a few per cent of rated load.
Droop arithmetic for kW sharing
Share of each set ∝ rating ÷ droop percent, at the common frequency
Worked example (illustrative)
Two sets: A is 500 kW with 4 percent droop, B is 1,000 kW with 4 percent droop. Total load 900 kW. Each takes in proportion to rating: A takes 300 kW (60 percent), B takes 600 kW (60 percent). If B’s droop is set at 3 percent, B takes more: ratios 500/4 : 1000/3 = 125 : 333, so A ≈ 246 kW (49 percent), B ≈ 654 kW (65 percent). The unequal loading needs correcting.
Frequency at load
With 4 percent droop, no-load frequency 51 Hz and full-load 49 Hz give 50 Hz at 50 percent load. Frequency varies with load unless a secondary control restores it.
Sharing quality check
| Load level | Acceptable difference in loading percent |
|---|---|
| 25 to 100 percent | Typically within 5 percent of rating between sets |
Troubleshooting
- Constant imbalance: droop or speed setpoint mismatch.
- Oscillation: governor gain too high or noisy load-share line.
- Reverse power on one set: speed setpoint too low relative to the others.
See kW and kVAr sharing and kW load-sharing control.
Common mistakes
- Setting one unit to droop and another to isochronous with no swing-unit plan.
- Ignoring the load-share line.
- Judging sharing by current rather than kW.
What to send us
Send the generator and controller models and current settings. Send your application details. See also kW load-sharing control.
Questions engineers ask
What is droop?
A governor characteristic where frequency falls as load rises, allowing parallel units to share load naturally.
Why do my generators not share evenly?
Common causes are different droop or gain settings, a faulty load-share line and governor differences.
Does the swing unit take all load changes?
In a one-isochronous scheme, yes, so it should have enough capacity and response for the swings.
How close should sharing be?
Specifications vary; a few per cent of rating is typical. Check the controller’s documentation.
Figures are illustrative. Use manufacturer settings and commissioning data.