The synchronization of an incoming alternator with a running system can be achieved using methods such as the "dark lamp" and "bright lamp" techniques. These methods utilize light bulbs connected between the generator terminals and the system terminals to provide visual feedback on frequency differences. Modern synchronization relies on microprocessors to automatically sense line voltage, frequency, phase sequence, phase angle, and waveform, ensuring precise connection to the grid. The historical use of lamps for manual synchronization has been largely replaced by automated systems.
PREREQUISITESElectrical engineers, technicians involved in power generation, and anyone interested in the synchronization of alternators with electrical grids.
http://en.wikipedia.org/wiki/Alternator_synchronization"...three light bulbs were connected between the generator terminals and the system terminals (or more generally, to the terminals of instrument transformers connected to generator and system). As the generator speed changes, the lights will rise and fall in intensity at a rate proportional to the difference between generator frequency and system frequency.