The grid has no battery in the middle. Supply must equal demand every instant, and the only real-time proof of that balance is a single number: the frequency. This is the control-theory machine that holds it at 60 Hz — droop, deadbands, reserve tiers, ROCOF, and the load-shedding relays of last resort — and the blackouts that happen when the machine loses the race.
Renewables
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Grid Frequency Regulation: The Control Loop That Never Sleeps