Three Dark-Lamp Method of Synchronizing a Generator to the Grid

Unlike DC generator, synchronous generator, which is the commonly used type of generator for power plants around the world, need to have the same frequency and in-phase to the grid frequency before connecting. This is also the reason why the act of interconnecting a synchronous generator to the Transmission Grid is synchronization.

If the generator output voltage frequency (usually 60 Hz and 50 Hz) is not the same and not in-phase with the Grid voltage frequency, there is a voltage difference between the two. Thus, interconnecting the two NOT in-phase voltage could be disastrous (a short circuit).

Since it is not possible to visually see the voltage frequency of both generator and the grid, a method called dark-lamp method could be used. In dark-lamp method, a lamp or bulb or anything the emit light is connected between the source of two frequencies. If the two frequencies, grid and generator, are the same and in-phase, there is no voltage difference and thus, the lamp will not emit light. That’s the only time that the generator can be connected to the Transmission Grid. A synchronoscope is also added in the synchronization set-up to help the plant operator in controlling the rotational speed of the generator.

Of course, dark lamp method is an old technology.

The following video is an actual synchronization of a hydro power generation unit to the grid.

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