Parallel Operation of Alternators Synchronizing

PARALLEL OPERATION OF ALTERNATORS SYNCHRONIZING

Parallel Operation

When an alternator is connected with another alternator (or) with common bus bars, then it is known as parallel operation.

Advantages of parallel operation

1. Continuity of supply can be maintained properly.

2. Repair and maintenance of individual generating units can also be done.

3. Maximum efficiency can be obtained and it is uneconomic to operate on low loads for large alternators.

4. Several small units are added depending upon their load requirements and can be operated at near to their rated capacity.

5. Initial capital cost of buying large units can be replaced by adding several small units in parallel.

6. The demand of high single power can be replaced.

Conditions for parallel operations

Proper synchronization of alternators is possible only by fulfilling the following conditions.

1. The terminal voltage of incoming alternator must be same as that of bus bar voltage.

2. The frequency of incoming alternator must be same as that of bus bar frequency.

3. The phase sequence of the voltage of incoming alternator should be same as that of bus bar voltage.

Methods of synchronization

1. Lamp dark method

2. Lamp bright method

3. Using synchroscope

Now we can look up these above methods in detail.

1. Lamp dark method

This is a method in which lamps are directly connected across synchronizing switch. In this operation, the voltages and frequencies of both systems must be made equal. The assumption is made such that the three lamps will be dark (or) bright together. Below certain voltage the lamp will be dark.

If dark is judged and switch is closed at middle then the machines are synchronised. If the phase sequence does not coincide then one lamp will be dark and other two will be bright and vice versa.

2. Lamp bright method

This is a method in which lamps are connected assymetrically as shown in Fig. 4.15(a). If the phase correct, the lamps brightens and glow dim in sequence.

Synchronisation is done at the moment the uncrossed lamp L₁ is in the middle of the dark period. When uncrossed lamp L₁ is dark, then other two crossed lamps L₂ and L₃ are dim but equally bright. This method of synchronising is known as 'two bright and one dark method.

3. By using synchroscope

Synchroscope is one which determines the instant of synchronism more accurately than the lamp methods. This synchroscope consists of a rotor and a stator, in which one is connected to the incoming alternator and other to bus bar. This is shown in Fig. 4.15(b). The pointer connected to rotor will rotate if there exists difference in frequencies of the incoming alternator and bus bar. Anticlockwise rotation of the pointer represents that frequency of alternator lesser than the busbar frequency. If pointer rotates in clockwise then it indicates frequency is higher than bus bar frequency. When frequencies are equal the speed of prime mover driving the alternator will have to be adjusted so that pointer is stationary.

By closing the switch the alternator can be switched on to the bus bar.