Three Dark Lamp Method of Synchronizing Alternator

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

Three Dark Lamp Method of Synchronising 

Three-Dark-Lamp-Method-Synchronising


As shown in 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 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 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 same and phase sequence of both is correct then voltage across three lamp will be zero. Hence lamps are completely  dark and this is right time to closed synchronizing switch. 

If the fequency 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 lamp is 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 grid and incoming alternator is not equal and other conditions are fulfilled then all lamps will equal bright and will continue to attain equal brightness.
 

Advantages of the Dark Lamp Method

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

Disadvantages of the Dark Lamp Method

  • 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.

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