Synchronization of Alternators by Synchroscope

Conditions Required for Paralleling Alternators

In order to connect an alternator in parallel with another alternator or an alternator to the infinite busbars, the following conditions are met −

• The phase sequence of the voltages of the incoming alternator should be the same as that of the busbars.

• The incoming alternator voltages must be in phase with the busbar voltages.

• The terminal voltage of the incoming alternator must be the same as the busbar voltage.

• The frequency of the generated voltage of the incoming alternator must be equal to frequency of the busbar voltage.

Synchronization by a Synchroscope

A stationary alternator must not be connected to live busbars. It is because the EMF induced by the alternator is zero at standstill and a short circuit will result. In order to check the synchronization of alternators, an equipment named Synchroscope is used.

Here, the phase sequence of the alternator is checked carefully at the time of its installation. The conditions 1 and 2 required for the synchronization are assured by means of the Synchroscope (shown in the figure). The Synchroscope compares the voltage from one phase of the incoming alternator with that of the corresponding phase of the 3-phase system.

The position of the pointer of the Synchroscope indicates the phase difference between the voltages of the incoming alternator and the infinite busbar.

• When the frequencies of the two voltages are equal, the pointer remains stationary.

• When the frequencies differ, the pointer rotates in one direction or the other.

The direction of the rotation of the pointer shows whether the incoming alternator is running too fast or too slow, i.e., whether the frequency of the incoming alternator is higher or lower than that of the infinite busbar. The speed of the rotation of the pointer is equal to the difference between the frequency of the incoming alternator and the frequency of the infinite busbar.

The frequency and phase positions are controlled by controlling the input to the prime mover of the incoming alternator. When the pointer of the Synchroscope moves very slowly, that is the two frequencies are almost same and passes through the zero-phase point, the circuit breaker is closed and the incoming alternator is connected to the busbar.

It is to be noted that the Synchroscope checks the relationships only on one phase and it does give any information about the phase sequence.