How can a power imbalance change grid frequency?

An AC grid's frequency is the rate at which its electrical waveform repeats. In a synchronous-generator area, generator rotation is linked to that frequency. A sudden difference between mechanical power entering the generators and electrical power demanded changes energy stored in their rotating masses.

If demand exceeds generation, that shortfall initially draws on rotational kinetic energy. The generators slow and frequency falls. A surplus has the opposite tendency. Aggregate inertia describes the rotating system's resistance to that speed change; it does not remove the energy imbalance.

This short-time approximation starts at 50 hertz on a 1,000-megawatt base. Its inertia constant H is stored rotational energy divided by rated power, measured in seconds rather than a waiting time. The stated initial relation is df/dt = f0ΔP/(2HSbase), with ΔP positive for excess generation.

For ΔP = −10 megawatts and H = 2.5 seconds, the slope is −0.1 hertz per second. Holding that slope for the illustrated four seconds gives 49.6 hertz. Doubling H halves the initial slope under the same imbalance. Equal generation and demand give no drift in this restricted model.

The falling frequency is a consequence of an energy shortfall, not energy disappearing from nowhere. Rotating machinery supplies some of its stored energy as it slows. Greater inertia slows that initial change and gives control responses more time, but it cannot supply an ongoing shortfall indefinitely. Governors, changing loads and other controls must change the power balance if continued drift is to stop.