Lecture Notes On Electrical Machines II (For Students B-Tech 4th Semester)

Parallel Operation of Alternators

The operation of connecting an alternator in parallel with another alternator or with common busbars is known as synchronizing. Generally, alternators are used in a power system where they are in parallel with many other alternators.

It means that the alternator is connected to a live system of constant voltage and constant frequency.

Often the electrical system, to which the alternator is connected, has already so many alternators and loads connected to it that no matter what power is delivered by the incoming alternator, the voltage and frequency of the system remain the same.

In that case, the alternator is said to be connected to infinite busbars.

It means that the switch must be closed at (or very near) the instant the two voltages have correct phase relationship. Condition (1) is indicated by a voltmeter, conditions (2), (3) and (4) are indicated by synchronizing lamps or a synchronoscope.

The synchronizing lamp method is consists of 3 lamps connected between the phases of the running 3-ph generator and the incoming generator as shown in Figure 1 above.

However, first it is necessary that the incoming alternator is correctly ‘phased out’ i.e. the phases are connected in the proper order of R, Y and B not R, B, Y etc. Lamp L₁ is connected between R and R′, L₂ between Y and B′ (not Y and Y′) and L₃ between B and Y′ (and not B and B′) as shown in Figure 2.

Two set of star vectors will rotate at unequal speeds if the frequencies of the two are different. If the incoming alternator is running faster, then voltage star R′ Y′ B′ appear to rotate anticlockwise with respect to the busbar voltage star RYB at a speed corresponding to the difference between their frequencies.

With reference to Figure 3, it is seen that voltage across L₁ is RR′ to be increasing from zero, and that across L₂ is YB′ which is decreasing, having just passed through its maximum, and that across L₃ BY′ which is increasing and approaching its maximum.

Hence the lamps will light up one after the other in the order 2, 3, 1, 2, 3, 1 or 1, 2, 3. If the incoming alternator is running slower, then the sequence of light up will be 1, 3, 2.

Synchronization is done at the moment the uncrossed lamp L₁ is in the middle of the dark period and other two lamps are equally bright. Hence this method of synchronization is known as two bright one dark lamp method.