Droop Control: Power Sharing Between Grid-Forming Sources

System Frequency

50.00 Hz

Microgrid Controls

Total Load 150 kW

Diesel Genset Droop 5.0 %

Inverters Droop 3.0 %

Generation Units

Diesel Genset

0.0 kW

Inverter A

0.0 kW

Inverter B

0.0 kW

Total Generation

0.0 kW

Freq. Deviation

0.00 Hz

How it works

Independent generation units on the same AC bus must agree on a single shared frequency without talking to each other. Droop control makes that possible: each unit's active-power output is a straight-line function of the bus frequency, so power splits automatically in proportion to how "stiff" each unit's line is.

Droop slope:  m_i = (R_i / 100) · f_nom / P_rated,i    [Hz per kW]
Unit output:  P_i(f) = clamp( (f_nom − f) / m_i ,  0, P_rated,i )
Bus balance:  P_load = Σ P_i(f)  →  solved for the common f
Dynamics:     dP_i/dt = (P_i(f) − P_i) / τ_i     (governor / inverter-loop lag)
              df/dt   = K · (ΣP_i − P_load)      (swing-equation approximation)

Droop % (R) — how much the frequency must sag, as a percent of nominal, for a unit to ramp from 0 to its full rated output. A smaller R means a stiffer, more aggressive responder that grabs a bigger share of any load change.
τ (time constant) — the diesel genset's mechanical governor is slow (~2.5 s); grid-forming inverters react almost instantly (~0.2 s), which is why they visibly catch the first swing of any load step before the diesel spins up.
Trip Diesel Offline — removes the genset from the droop pool; the two inverters alone must now cover the whole load, so the steady-state frequency sags further for the same load if their combined droop can't match it.

Real-world relevance: this is exactly how island microgrids, ship power plants, and modern grid-forming battery inverters keep multiple sources synchronized and sharing load fairly — no central controller, just each unit obeying its own droop curve.

Common bus Diesel genset Inverter A Inverter B

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