Generator Droop and Load Sharing Formulas
Generator droop formulas: the droop percentage, the load from the frequency and the load shared by parallel generators with the same droop.
The formulas
| To find | Formula |
|---|---|
| Speed droop | droop % = (fnl − ffl) / ffl × 100 |
| Load fraction at frequency f | x = (fnl − f) / (fnl − ffl) |
| Bus frequency for a total load | f = fnl − (Ptotal / ΣPrated) × (fnl − ffl) |
| Load on each unit | Pi = x × Prated,i |
fnl is the frequency at no load and ffl the frequency at full load, so a unit with a droop governor lowers its frequency as the load rises. Generators in parallel on the same bus share the same frequency. If they have the same droop and the same no-load frequency, they share the load in proportion to their ratings. A different droop or setpoint changes the share, and the operator or the load sharing control then moves the setpoints to correct it. The reactive power shares by the voltage regulation in the same way, which this page does not cover. The values here are examples, and the maker gives the governor settings.
Worked example: no load 52 Hz, full load 50 Hz
- droop = (52 − 50) / 50 × 100 = 4%
- At 51 Hz, x = (52 − 51) / (52 − 50) = 0.5, which is half of the rating
Worked example: units of 500 kW and 1,000 kW sharing 600 kW
- x = 600 / (500 + 1000) = 0.4
- f = 52 − 0.4 × (52 − 50) = 51.2 Hz
- P1 = 0.4 × 500 = 200 kW and P2 = 0.4 × 1000 = 400 kW
Frequency by load share (52 Hz at no load, 50 Hz at full load)
| Load | Frequency |
|---|---|
| 25% | 51.5 Hz |
| 50% | 51 Hz |
| 75% | 50.5 Hz |
| 100% | 50 Hz |
Frequently asked questions
Why do the generators need a droop?
The droop makes the frequency fall as the load rises, which gives each unit its own point on the line and lets two units share the load without fighting each other.
What happens with a different droop?
The unit with the smaller droop takes a larger share of a change in load, so the shares no longer follow the ratings. Match the droop to share in proportion.
What does an isochronous governor do?
It holds the frequency constant. Only one unit on a bus should run isochronous, and the others run on droop, or a load sharing control links them.
How are the shares adjusted?
The setpoint of the governor moves the droop line up or down, which raises or lowers the load of that unit at the same frequency.
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