Improper Synchronisation has disastrous consequences. In case there is a mismatch in Phase Sequence, it is as good as Short Circuit.
Synchronisation means matching the Parameters of Generator with Grid before connecting it to Grid.
Three Dark Lamp Method is one of the Method for synchronising. Three Lamps are connected across the Switch. Generator Frequency is brought very near to Grid Frequency. Beats are formed across the Switches. Lamps become Dark and Light up due to Beats. Switches are turned on when Lamps are Dark.
In case Lamps light up one after another, it indicates that Phase Sequence of Generator is not correct. Any two terminals of Generator are changed.
In order to find out whether Incoming Generator is too slow or too fast, one Lamp is connected between Y Phase of Grid and B Phase of Generator. Second Lamp is connected between B phase of Grid and Y Phase of Generator. Sequence of Light up of Bulbs indicate whether Incoming Generator is too slow or too fast.