Grid Inertia and RoCoF
How aggregate inertia shapes frequency slope, nadir, and load-shedding margin after a contingency.
Grid Inertia and RoCoF
How aggregate inertia shapes frequency slope, nadir, and load-shedding margin after a contingency.
Control visualization - autoplay the generation-loss event, scrub the timeline, or adjust inertia and lost generation to see RoCoF and nadir move.
What it shows
When generation is suddenly lost, frequency falls. The initial slope — the rate of change of frequency (RoCoF) — is set by how much rotational inertia the system has. Lower inertia means a steeper initial drop and a deeper nadir, risking under-frequency load shedding (UFLS). The curve shows how inertia and fast response together determine whether frequency recovers before the UFLS threshold is crossed.
Why it matters for BESS
Inverter-based resources do not inherently provide inertia, but a BESS can deliver fast frequency response (and synthetic inertia) within milliseconds — flattening RoCoF and raising the nadir. This is one of the highest-value grid services storage provides as conventional inertia declines.
Frequently asked
- What is RoCoF?
- RoCoF is the rate of change of frequency (Hz/s) immediately after a power imbalance. It is inversely proportional to system inertia: less inertia gives a faster, more dangerous frequency excursion.
- Can battery storage provide inertia?
- Not true mechanical inertia, but grid-forming and fast-response BESS can emulate it (synthetic inertia) and deliver fast frequency response far quicker than governors on conventional plant, which arrests the frequency decline.