In short. Real power (kW) sharing between paralleled generators is set by their engine governors. In droop mode each set’s frequency falls slightly with load, so sets on the same bus settle where their droop lines meet the common frequency. In isochronous mode a load-sharing link holds frequency constant while the controllers divide the load. The goal is that every running set carries the same percentage of its own rating; when one runs hotter than the others, the governor settings or the link are usually the cause.
Who this page is for: For commissioning engineers, plant operators and consultants checking or troubleshooting kW sharing between generators.
Key facts
- Droop (%) = (no-load frequency − full-load frequency) ÷ rated frequency × 100.
- With droop, each set’s load is proportional to its rating divided by its droop.
- Equal droop and equal no-load frequency give proportional sharing, whatever the set sizes.
- Typical droop is 3 to 5 percent; 51.5 Hz at no load and 50 Hz at full load is 3 percent.
- Isochronous sharing holds 50 Hz but needs the load-share link and a fallback.
- Good sharing is usually within a few percent of rating.
How droop sharing works
Because every set runs at the same bus frequency, each settles where its own droop line crosses that frequency. Sets with the same droop and no-load setting therefore carry the same fraction of their rating. A speed trim or secondary controller can then bring the bus back to 50 Hz without upsetting the shares.
Worked example: mixed sizes, mismatched droop (illustrative)
A 1000 kVA set (800 kW) and a 500 kVA set (400 kW) share a 900 kW load, with the same no-load frequency.
| Case | Working | Large set | Small set |
|---|---|---|---|
| Both at 4 percent droop | Share in proportion to rating | 600 kW (75 percent) | 300 kW (75 percent) |
| Large at 4 percent, small at 3 percent | 0.04 × PL ÷ 800 = 0.03 × PS ÷ 400, PL + PS = 900 | 540 kW (67.5 percent) | 360 kW (90 percent) |
A one-point difference in droop pushes the small set from 75 to 90 percent while the large one runs lighter. Any further load brings the small set to full load first. Matching the droop settings, or using isochronous sharing, restores equal loading. See multi-DG load sharing.
Do not judge sharing by total kW. Compare each set’s kW as a percentage of its own rating, at several load levels, and correct any set that consistently runs higher.
Sharing modes compared
| Mode | Frequency | How load is divided | Notes |
|---|---|---|---|
| Droop | Falls with load | Natural, from droop lines | Simple, robust; needs speed trim for 50 Hz |
| Isochronous with load-share link | Constant | Controllers exchange load data | Standard in modern plants; needs a fallback |
| One isochronous, others droop | Constant | The isochronous set takes all changes | That set must have the capacity and response |
| Base load (with the grid) | Set by the grid | Fixed kW per set | Used in utility parallel |
Typical causes of poor sharing
- Different droop or gain settings.
- A faulty or disconnected load-share link.
- A sticking actuator or governor fault on one engine.
- Mixed set sizes without proportional settings.
- Measurement errors from CT ratio or wiring mistakes.
Commissioning checks
- Record kW, kVAr and current of each set at about 25, 50, 75 and 100 percent load.
- Compare each set’s kW as a percentage of its rating.
- Step the load and watch for hunting between sets.
- Break the load-share link and confirm a safe fallback.
Use a load bank where site load is not enough.
Sharing test record
| Test load | Set 1 kW (% of rating) | Set 2 kW (%) | Set 3 kW (%) | Frequency (Hz) |
|---|---|---|---|---|
| 25% | ||||
| 50% | ||||
| 75% | ||||
| 100% |
Common mistakes
- Different droop settings on sets in the same plant.
- Comparing kW instead of percentage of rating.
- No fallback when the load-share link fails.
- Testing at one load level only.
What to send us
Send each set’s rating, governor settings and readings of kW, kVAr and current at several loads. WhatsApp 98852 01608 or use the enquiry form. See also kVAr sharing.
Evidence note. Droop values are typical for illustration; governor and controller makers’ settings govern.
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 percent of rating is typical. Check the controller’s documentation.
Figures are illustrative. Use manufacturer settings and commissioning data.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators. Last updated 8 November 2026.