Ah, then you're into mechanics. Excess power is continuously converted into increased rotational speed of a very large turbine at a hydro power plant. But the i

Ah, then you're into mechanics. Excess power is continuously converted into increased rotational speed of a very large turbine at a hydro power plant. But the increase in speed takes some time due to inertia. The hydro plant operator has a device attached to the turbine, a regulator, that reduces the flow of water running through the turbines in real-time - lowering the amount of power going into the system which in turn slows down the turbine. This service is the foundation of a working power system and is called "automatic" or "primary" regulation. Usually the system operator pays ahead of time for this service to be available followed by an additional payment afterwards for actual use. A single hydro plant can only do so much though - it can not reduce the flow of water below zero and the system will fail if it ever runs out of primary regulation reserves. The system operator will then have to pay an additional party to produce less in order to prevent that from happening. This is called "secondary" or "tertiary" regulation and is not real-time.