Which source could be counted as dynamic reactive reserve from a rotating source?

The correct answer, which is a generator in automatic voltage regulation, is considered a dynamic reactive reserve from a rotating source because it can quickly respond to changes in the system and provide reactive power. When a generator operates under automatic voltage regulation (AVR), it monitors the system voltage and adjusts its output accordingly. This capability allows it to react promptly to voltage fluctuations, maintaining stability within the electric grid.

Generators that operate in this way contribute to dynamic reactive power support, which is essential during disturbances or fluctuations in load. They can help maintain voltage levels and enhance system reliability, making them a vital resource in managing reactive power requirements in real-time.

In contrast, static VAR compensators are primarily static devices that provide reactive power support without the rotational inertia characteristic of generators, and therefore do not qualify as dynamic reactive reserves. Battery storage systems, while capable of offering many services, do not typically provide reactive power unless specially designed and controlled to do so. Shunt reactors can absorb reactive power but are also static devices lacking the dynamic response capability of a rotating source.