Methods of Synchronization
Methods of Synchronization:
The method of connecting an incoming alternator safely to the live busbars is called synchronizing. The equality of voltage between the incoming alternator
and the busbars can be easily checked by a voltmeter. The phase sequence of the alternator and the busbars can be checked by a phase sequence indicator. Differences in frequency and phase of the voltages of the incoming alternator and busbars can be checked by one of the following two methods:
(i) By Three Lamp (one dark, two bright) method
(ii) By synchroscope
(i) Three lamp method:
In this method of synchronizing, three lamps L1, L2 and L3 are connected as shown in Fig. (1). The lamp L1 is straight connected between the corresponding phases (R1 and R2) and the other two are cross-connected between the other two phases. Thus lamp L2 is connected between Y1 and B2
and lamp L3 between B1 and Y2. When the frequency and phase of the voltage of the incoming alternator is the same as that of the busbars, the straight connected lamps L1 will be dark while cross-connected lamps L2 and L3 will be equally bright. At this instant, the synchronization is perfect and the switch of the incoming alternator can be closed to connect it to the busbars.
FIg: 1 Fig: 2
In Fig. (2), phasors R1, Y1 and B1 represent the busbars voltages and phasors R2, Y2 and B2 represent the voltages of the incoming alternator. At the instant when R1 is in phase with R2, voltage across lamp L1 is zero and voltages across lamps L2 and L3 are equal. Therefore, the lamp L1 is dark while lamps L2 and L3 will be equally bright. At this instant, the switch of the incoming alternator can be closed. Thus incoming alternator gets connected in parallel with the busbars.
A synchroscope is an instrument that indicates by means of a revolving pointer the phase difference and frequency difference between the voltages of the incoming alternator and the busbars [See Fig. (2)]. It is essentially-a small motor, the field being supplied from the busbars through a potential transformer and the rotor from the incoming alternator. A pointer is attached to the rotor. When the incoming alternator is running fast (i.e., frequency of the incoming alternator is higher than that of the busbars), the rotor and hence the pointer moves in the clockwise direction. When the incoming alternator is running slow (i.e., frequency of the incoming alternator is lower than that of the busbars), the pointer moves in anti-clockwise direction. When the frequency of the incoming alternator is equal to that of the busbars, no torque acts on the rotor and the pointer points vertically upwards (“12 O’ clock”). It indicates the correct instant for connecting the incoming alternator to the busbars. The synchroscope method is superior to the lamp method because it not only gives a positive indication of the time to close the switch but also indicates the adjustment to be made should there be a difference between the frequencies of the incoming alternator and the busbars.