Synchronization (alternating current)

If two unconnected segments of a grid are to be connected to each other, they cannot safely exchange AC power until they are synchronized.

However, an AC generator must additionally match its timing (frequency and phase) to the network voltage, which requires both speed and excitation to be systematically controlled for synchronization.

[2] The voltage, frequency and phase angle must be controlled each time a generator is to be connected to a grid.

[1] Generating units for connection to a power grid have an inherent droop speed control that allows them to share load proportional to their rating.

The generator is brought up to approximate synchronous speed by supplying more energy to its shaft - for example, opening the valves on a steam turbine, opening the gates on a hydraulic turbine, or increasing the fuel rack setting on a diesel engine.

In this scheme, when the generator was in synchronism with the system, one lamp would be dark, but the two with the swapped connections would be of equal brightness.

Another manual method of synchronization relies on observing an instrument called a "synchroscope", which displays the relative frequencies of system and generator.

Synchroscopes or lamps are sometimes installed as a supplement to automatic relays, for possible manual use or for monitoring the generating unit.