Three Dark Lamp Method of Synchronizing Alternator

The procedure of connecting an alternator to the infinite power grid or with a common bus to which the number of other generators are already connected in parallel is known as synchronizing of the alternator.  Here we are going to discuss three Dark Lamp Method of alternator synchronizing.

Three Dark Lamp Method of Synchronising

As shown in the diagram alternator A is already connected to the infinite bus or grid. The other alternator named with B is to be synchronized with grid or bus by using the three dark lamp method.

Let lamps L1, L2 and L3 are connected across R-R’, Y-Y’ and B-B’ terminals of synchronizing switch, here R, Y, and B are the terminals of the infinite bus bar, and R’, Y’ and B’ are the terminals of incoming alternator B.

When voltages are equal and the frequencies of alternator B and Bus is the same and the phase sequence of both is correct then the voltage across three lamps will be zero. Hence lamps are completely dark and this is the right time to close synchronizing switch.

If the frequency of the alternator B is different than grid frequency and other conditions are satisfied then the lamps will flicker simultaneously.

The frequency of flickering of the lamp depends upon the frequency difference between incoming alternator B and the grid frequency fb-fa.

If the phase sequence of alternator B and grid is not correct then the lamps will flicker one by one. Then the phase sequence is corrected by interchanging any two leads of incoming alternator B.

The flickering of lamps L1, L2, and L3 are reduced by varying the speed of the prime mover of the incoming alternator.

If the voltage of the grid and the incoming alternator is not equal and other conditions are fulfilled then all lamps will be equally bright and will continue to attain equal brightness.

Advantages

• 3 dark lamp method is cheaper.
• The correct phase sequence can be easily identified.

Disadvantages

• The lamp becomes dark at about half of its rated voltage. Hence, it is possible that the synchronizing switch might be closed even when there is a phase difference present between the grid and incoming alternator.
• The filament of the lamp might burn out.
• the flicker of the lamps does not show which lamp has the higher frequency.