In this topic, you study Dark Lamp Method of Synchronization of Generator (or Alternator).
Fig. 4.1 shows the necessary circuit diagram for synchronizing three phase alternators by this method.
Fig. 4.1: Synchronizing of three-phase alternators using ‘Three dark lamps’ method
Here, let us assume that Alternator-I is already connected to the bus-bars and Alternator-2 is to be synchronized i.e. put in parallel with Alternator-I for the purpose Of sharing the common load.
Procedural Steps for Dark Lamp Method:
The procedural steps for this will be as follows .
Step-1 : Prime mover of Alternator-2 is started and its speed is adjusted to the rated value.
Step-2 : By adjusting the field current, the terminal voltage of the incoming alternator is made equal to that of the bus-bars to which Alternator-I is already connected. This can be done
by comparing the readings Of the voltmeters connected to the incoming alternator and bus-bar sides.
Step-3 : By controlling the speed of the prime mover, the frequency of the incoming alternator is also made equal to bus-bar frequency. Frequency meters are connected to the incoming alternator and bus-bar sides for this purpose.
Step-4 : Phase sequence of the incoming alternator and the bus-bars is then checked with the help Of a phase sequence tester. The commonly used phase sequence tester is nothing but a miniature three-phase induction motor (Fig. 4.2) having a rotating disc marked with an arrow. If this disc rotates in the same direction (either clockwise or anticlockwise) when the phase sequence tester is connected in turn to the incoming alternator and bus-bar sides, it indicates that the phase sequence Of the incoming alternator and bus-bars is the same. If the phase sequence is not matching, it can be adjusted by interchanging any two terminals of the incoming alternator.
Fig. 4.2 : Phase sequence tester
Step-5 : Finally the instant of synchronizing i.e. the exact instant when the voltages of the incoming alternator and bus-bars are in phase (with respect to the load circuit) is determined by the use of three identical incandescent lamps connected across the synchronizing switch as shown in Fig. 4.1. It will be observed that these lamps flicker at a rate decided by the difference in frequencies Of the incoming alternator and bus-bars. All these lamps will be becoming bright together and dark together. Small adjustment in the speed of the prime mover of the incoming alternator is then made until the lamps flicker at a very low rate. The synchronizing switch is then closed at the instant when all the three lamps are dark.
Explanation of Dark Lamp Method
Fig. 4.3 shows the relevant phasor diagram for this method under the condition when small frequency difference exists between the frequencies Of the incoming alternator and bus-bars.
Fig. 4.3 : Phasor diagram for ‘Three dark lamps’ method
Here, the phasors Vr(bus) , Vy(bus) and Vb(bus) represent the bus-bar phase voltages. Similarly Vr(alt) , Vy(alt) and Vb(alt) represent the phase voltages of the incoming alternator. These two sets of voltages rotate in anticlockwise direction at unequal speeds when the frequencies of the incoming alternator and bus-bars are different. The voltages across the lamps L1, L2 and L3, are shown by the phasors VL1, VL2 and VL3. Each of this voltage is obviously equal to difference between the corresponding two phase voltages. From this phasor diagram, it will be clear that the voltage across the lamps varies from zero to twice the phase voltage (the lamps must be rated for this higher voltage or series combination of two lamps of standard voltage ratings may be used in place of one lamp of higher voltage rating) causing the lamps to flicker.
At the instant when the incoming alternator voltage and bus-bar voltage will be in phase, the voltage across each lamp will be zero. Therefore, they will be dark under this condition. The synchronizing switch should be closed at this instant.
Advantages of Dark Lamp Method
(i) Needs inexpensive equipment.
(ii) proper phase sequence can be readily obtained using the three lamps. When all the
lamps become bright together and dark together, it indicates that the sequence of
the incoming alternator is same as that of the bus-bars.
Disadvantages of Dark Lamp Method
(i) Lamps become dark even with some voltage across them which is sufficiently less than their rated voltage. Hence, there is possibility of synchronizing switch being closed at the wrong instant. High circulating current caused under this condition can damage the alternators.
(ii) The flicker of the lamps does not indicate whether the incoming alternator is running faster or slower than the other alternator/s already connected to the bus-bars.
(iii) Lamp filaments can burn out, leading to a wrong choice of the instant of synchronizing.
