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 by default (~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. Both are adjustable here.
- Grid stiffness K — how fast the shared bus frequency itself moves in response to a generation/load mismatch (a stand-in for total system inertia). Larger K means a jumpier, less inertial grid.
- 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.
The right-hand droop-line chart plots each online unit's straight droop line (output vs. frequency); the dot on each line is that unit's live operating point, and the vertical dashed line is the current shared frequency — exactly where all the lines must agree. The strip chart below tracks that shared frequency over time so you can see the diesel's slow catch-up trailing the inverters' fast one after any change.
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.