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Synchronization of Alternators by Synchronizing Lamps Method
A set of three synchronizing lamps can be used to check the conditions for paralleling the incoming machine with other machines. The dark lamp method along with a voltmeter used for synchronising is shown in figure. This method is used for synchronizing low-power machines.
In this method, one lamp is connected between corresponding phases while the other two lamps are cross-connected between the other two phases, i.e., R1 is connected to R2, Y1 to B2 and B1 to Y2 as shown in the figure.
Now, the prime mover of the incoming machine is started and the alternator is brought up to near its rated speed. The excitation of the incoming machine is to be adjusted until the induced voltages $𝐸_{𝑅2}, 𝐸_{𝑌2}, 𝐸_{𝐵2}$ of the incoming machine are equal to the busbar voltages $𝑉_{𝑅1}, 𝑉_{𝑌1}, 𝑉_{𝐵1}$.
The three lamps flicker at a rate equal to the difference in the frequencies of the incoming machine and the busbar. The frequency of the incoming machine is adjusted until the lamps flicker at a very slow rate.
The right moment to close the synchronising switches is obtained at the instant when the straight-connected lamp is dark and the cross-connected lamps are equally bright. If the phase sequence is incorrect, then all the lamps will be dark simultaneously.
In order to correct the phase sequence, the two leads of the line of the incoming machine should be interchanged. Since the dark range of a lamp extends over a considerable voltage range, a voltmeter V is connected across the straight connected lamp and the synchronizing switch is closed when the voltmeter reading is zero. The incoming machine is now floating on the busbars and ready to take up the load as a generator.
Advantages of Dark Lamp Method
The proper phase sequence can be easily determined.
This method of synchronization is less expensive.
Disadvantages of Dark Lamp Method
Following are the disadvantages of the dark lamp method −
The lamp filament might burn out.
The lamps may become dark at about half their rated voltage, thus it is possible that the synchronising switch might be closed when there is a considerable phase difference between the machine and the busbars. This may result in high circulating current to damage the machine.
The flicker of the lamps does not indicate which machine has the higher frequency.
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