Synchronization of Alternators

For proper synchronization of alternators, the following conditions must be fulfilled:

  • The phase sequence of the incoming alternator should be same as that of bus bars.
  • The terminal voltage of the incoming alternator should be equal to that of bus bar voltage.
  • The speed of incoming alternator should be such that its frequency is equal to that of bus bar frequency.

Phase Sequence

It can be checked by a sequence tester or with the help of a small induction motor.
Connect a small induction motor with bus bars and incoming alternator one by one. If induction motor is running in the same direction in both cases then the phase sequence is same.

Terminal Voltage

Fig. shows the connection diagram for the synchronization of alternators. The terminal voltage of incoming alternator can be adjusted by its field current. It will be equal to that of bus bars when lamp L1 will be off.

synchronization of alternators



To adjust frequency, the speed of incoming alternator is adjusted.
If the frequency of the incoming machine is not brought up to that of bus bars, then there will be a phase difference between their voltages (even when they are equal in magnitude). This phase-difference will be continuously changing with the changes in their frequencies.
Sometimes the resultant voltage is maximum and some other times minimum. Hence, the current is alternatingly maximum and minimum.
Due to this changing current through the lamps, a flicker will be produced. Lamps will dark out and glow up alternately.
Darkness indicates that the voltage of the incoming alternator is in exact phase opposition and hence there is no resultant current through the lamps.
Therefore, we have to adjust the speed of incoming alternator to get the co-phase condition.
When the co-phase condition arises (i.e. voltage and frequency of incoming alternator is equal to that of bus bars) lamp L1 will be off and lamp L2 and lamp L3 will glow with same intensity.
At this instant, synchronization switch should be closed.

synchronization of alternators image

After closing the switch, the alternator gets synchronized.

Synchronization of Alternators By Synchroscope

Synchroscope is an instrument used for synchronization of alternators. It indicates whether the incoming machine is running fast or slow.
By adjusting the speed of the incoming machine, its pointer is made stationary in the vertical position. And when this condition is achieved, the synchronization switch is closed.

synchronization of alternators by synchroscope


Load Sharing in Parallel Operation of Alternators

There are two loads on an alternator,

  • the apparent load (KVA)
  • the real load (KW).

The former is determined by the excitation while the latter is determined by the input to the prime mover.
The effective load (KW) delivered by an alternator can be regulated by regulation of power input to the prime mover. Only increased input will make the machine deliver more load.
Any change in excitation of an alternator operating in parallel with other machines has no effect upon the effective load (KW) delivered by that machine. It merely affects the power factor of the machine.
If the excitation is reduced sufficiently, the machine will deliver power at leading power factor.

Infinite Bus-bars

The large grid systems which are fed by numerous alternators of very large capacity are termed as Infinite Bus-bars.

Control of Bus-bar Voltage

When a number of alternators are running in parallel, the bus-bar voltage can be regulated by regulating the excitation of all the machines. Change of excitation of any one machine will only affect the reactive load delivered by the machine.

Control of Bus-Bar Frequency

When a number of alternators are running in parallel, the bus-bar frequency can be regulated by regulating the power input to all the machines. An increase of power input to any one machine will merely cause it to take more load at the expense of the other machines with the possibility of damage to itself.

Disconnection of Alternator Running in Parallel

To shut down an alternator running in parallel,

  • the KW load should be removed first by gradually reducing the power input to it
  • the reactive load should be removed by reducing the excitation

And the main switch should be opened when the alternator is merely floating on the bus-bars.


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