Primary response ≈ — synchronous plant, and it falls as you retire it.
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What sets the fall
Grid frequency is the tachometer of the whole system: 50 Hz means generation and demand match. Lose a big infeed and frequency falls — and how fast it falls in the first instant is set by inertia, the kinetic energy stored in spinning synchronous plant. The swing equation makes it concrete: RoCoF ≈ loss ÷ inertia. Swap synchronous plant for inverter-based wind, solar and batteries and that inertia drops — GB's operating floor fell to 120 GVA·s in 2024, from roughly 300 a decade earlier — so the same trip falls faster and deeper.
A battery earns its money by being fast. Dynamic Containment, a post-fault service that ramps to full output by ±0.5 Hz within a second, arrests the fall and lifts the nadir. Grid-forming units go further: as voltage sources they give an inertia-like response that flattens the initial slope itself — the part fast response can't reach. Different products for different moments of the fall.
This is a single-bus teaching model — one uniform system frequency, no locational effects — with illustrative parameters you can move, not a validated study of any real grid. It settles off-nominal, at a droop offset; full restoration to 50 Hz is secondary response over minutes, off-screen. Grid-forming inertia saturates at the inverter's current limit, unlike true spinning mass. And load damping — modelled here — is itself fading as demand becomes power-electronic, the same story as inertia.
The revenue is real. NESO's three Dynamic services are all here. Dynamic Regulation and Dynamic Moderation are pre-fault — they reach full output by ±0.2 Hz, DR slowly (10 s) and DM fast (1 s) — so in a large fall they saturate early and DR barely delivers before the nadir. Dynamic Containment is the post-fault service: it alone keeps responding past ±0.2 Hz, to full output at ±0.5 Hz — which is why containment-scale volume is what holds a deep event. Grid-forming is separate again: it's synthetic inertia, not a frequency-response service. NESO procures the Dynamic trio for response and, separately, inertia and stability through the Long-Term 2029 tender, successor to the Stability Pathfinders. Grid-forming itself was codified in GB's Grid Code (GC0137, a 2022 world-first) and is growing, though still a minority of contracted inertia today. Note the toy models demand disconnection below 48.8 Hz (UFLS), which is a different mechanism from RoCoF loss-of-mains protection.