Which type of reserve allows a subsystem to recover quickly from contingencies?

A synchronous reserve is designed to help a power system maintain stability and recover quickly from contingencies, such as unexpected outages or sudden increases in load. This type of reserve operates in real-time, enabling generating units to respond immediately to frequency changes caused by these contingencies. Synchronous reserves engage resources that can quickly provide power, thus stabilizing the system and preventing cascading failures or blackouts.

The effectiveness of synchronous reserves lies in their ability to match supply with demand almost instantaneously, ensuring that the system remains balanced despite sudden disturbances. By maintaining a stable frequency, synchronous reserves are crucial for the overall reliability of the transmission network, allowing it to recover swiftly and effectively from unexpected events.