In the last article, we have seen the synchronization of alternators by using the lamps method (dark lamp method, bright lamp method, and two bright and one dark method). The drawback of the lamps method (used for low voltage alternators) can be overcome by the synchroscope synchronization method. So that high voltage alternators can be synchronized and put under parallel operation.
A synchroscope is a device with a pointer rotating on a dial. The pointer indicates the phase relation between two alternators by its direction of rotation and also tells whether the incoming alternator is running faster or slower than the existing alternator.
The following must be the essential conditions for synchronization,- The generated terminal voltage of the incoming alternator must be equal to the existing alternator or bus-bar voltage.
- The frequency at which the voltage is generated must be also equal
- There must be zero phase difference with equal phase sequences.
Method of Synchroscope Synchronizing :
A synchroscope consists of a rotor with a moving coil to which the pointer is attached and a fixed coil that remains stationary. One coil is connected to any two phases of the incoming alternator and another coil is to any two phases of the bus-bars or existing alternator as shown in the below figure.
At first, the switch is kept open and if the frequencies of two alternators are different then there will be a movement in the pointer placed on the moving coil. The rotational movement is in such a way that,
- If the frequency of the incoming alternator is low compared to the bus-bars or existing alternator then the rotation is in the anti-clockwise direction.
- If the frequency of the incoming alternator is more, then the rotation is in the clockwise direction.
If the difference in frequencies is more the speed of rotation is also more. By adjusting the driving speed of the incoming alternator the frequency can be adjusted i.e., by adjusting the speed of the prime mover.
The pointer remains stationary in a vertical position when both the frequencies are made equal. Thus the switch S is closed at this instant and the incoming alternator is made to run parallelly with other alternators.
Machine Floats on the Bus-bar :
When the alternator is synchronized, at an instant the induced emf in the incoming alternator becomes equal to the bus-bar operating voltage. At this condition, the alternator is not supposed to take any load on it even though it is synchronized. This condition is called alternator floating on the bus-bar. There will be no load current supplied by the incoming alternator and the prime mover driving the alternator is to supply no-load losses only.
The floating of alternators is opposed by a synchronizing torque developed due to circulating current flowing through the alternators. The circulating current is developed due to some disturbances that cause to speed up the alternator. This results in delivering power to the other alternators by this alternator, thereby having a loading effect on this alternator.
The loading effect decreases the speed of the alternator. When the speed decreases there will be a power flow in the reverse direction i.e., to this alternator from other alternators and the synchronizing torque developed will tend to keep the alternator in synchronism with the others.