In an alternating current (AC) electric power system, synchronization is the process of matching the frequency, phase and voltage of a generator or other source to an electrical grid in order to transfer power. If two unconnected segments of a grid are to be connected to each other, they cannot safely exchange AC power until they are synchronized.
A direct current (DC) generator can be connected to a power network simply by adjusting its open-circuit terminal voltage to match the network's voltage, by either adjusting its speed or its field excitation. The exact engine speed is not critical. 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. This extra complexity was one of the arguments against AC operation during the war of currents in the 1880s. In modern grids, synchronization of generators is carried out by automatic systems.