GridEd · Module 5

Frequency, Inertia & Droop

Frequency
60.00 Hz
RoCoF (rate of change)
0.00 Hz/s
Nadir (lowest point)
60.00 Hz
Reserve deployed
0 MW

The grid is balanced at 60 Hz. Trip a generator and watch how far and how fast frequency falls — then change the grid's inertia and try again.

RoCoF = df/dt = −ΔP · f₀ ÷ (2·H·S) = 0.00 Hz/s
now: ΔP = 0 MW  ·  H = 4.6 s  ·  system = 1000 MW
Primary droop: Δfsettle ≈ −ΔP ÷ (governor stiffness)
Inertia (spinning mass) sets how fast frequency falls the instant supply and demand diverge — halve the inertia and the initial slope (RoCoF) doubles. Governor droop then arrests the fall and reserves refill the gap; fast frequency response (batteries, grid-forming inverters) acts in a fraction of a second to lift the nadir. If frequency falls too far, under-frequency relays shed load — the grid's last line of defense.

Teaching notes

  • The core contrast: with a thermal (high-inertia) grid, trip a big unit — frequency dips gently and recovers. Slide the mix toward inverter-based (low inertia) and trip the same unit — RoCoF is far steeper, the nadir deeper, and load-shedding relays fire.
  • RoCoF is the headline number: it scales as 1/H. This is exactly why system operators now worry about inertia as coal and gas retire.
  • Fast frequency response (batteries / grid-forming inverters / "synthetic inertia") acts in <1 s to arrest the nadir — flip it on in the low-inertia case and watch the dip shrink.
  • Turn droop off to show what governors do: with no primary response, frequency keeps sliding until relays dump large blocks of load.
frequency dashed = 60 Hz · red bands = load-shedding thresholds · dot = nadir
30% inverter · H=4.6 s
150 MW