Parallel operation of an alternator

Alternator is really an AC generator. In alternator, an EMF is induced in the stator (stationary wire) with the influence of rotating magnetic field(rotor) due to Faraday’s law of induction. Due to the synchronous speed of rotation of field poles, it is also known as synchronous generator.
Here, we can discuss about parallel operation of alternator. When the AC power systems are interconnected for efficiency, the alternators should also have to be connected in parallel. There will be more than two alternators connected in parallel in generating stations.

Condition for Parallel Operation of Alternator

There are some conditions to be satisfied for parallel operation of the alternator. Before entering into that, we should understand some terms which are as follows.

• The process of connecting two alternators or an alternatorand an infinite bus bar system in parallel is known as synchronizing.
• Running machine is the machine which carries the load.
• Incoming machine is the alternator or machine which has to be connected in parallel with the system.

The conditions to be satisfied are

1. The phase sequence of the incoming machine voltageand the bus bar voltage should be identical.
2. The RMS line voltage (terminal voltage) of the bus bar or already running machine and the incoming machine should be the same.
3. The phase angle of the two systems should be equal.
4. The frequency of the two terminal voltages (incoming machine and the bus bar) should be nearly the same. Large power transients will occur when frequencies are not nearly equal.

Departure from the above conditions will result in the formation of power surges and current. It also results in unwanted electro-mechanical oscillation of rotor which leads to the damage of equipment.

General Procedure for Paralleling Alternators

The figure below shows an alternator (generator 2) being paralleled with a running power system (generator 1). These two machines are about to synchronize for supplying power to a load. Generator 2 is about to parallel with the help of a switch, S1. This switch should never be closed without satisfying the above conditions.

1. To make the terminal voltages equal. This can be done by adjusting the terminal voltage of incoming machine by changing the field current and make it equal to the line voltage of running system using voltmeters.
2. There are two methods to check the phase sequence of the machines. They are as follows
   • First one is using a Synchroscope. It is not actually check the phase sequence but it is used to measure the difference in phase angles.
   • Second method is three lamp method (Figure 2). Here we can see three light bulbs are connected to the terminals of the switch, S1. Bulbs become bright if the phase difference is large. Bulbs become dim if the phase difference is small. The bulbs will show dim and bright all together if phase sequence is the same. The bulbs will get bright in progression if the phase sequence is opposite. This phase sequence can be made equal by swapping the connections on any two phases on one of the generators.

3. Next, we have to check and verify the incoming and running system frequency. It should be nearly the same. This can be done by inspecting the frequency of dimming and brightening of lamps.
4. When the frequencies are nearly equal, the two voltages(incoming alternator and running system) will alter the phase gradually. These changes can be observed and the switch, S1 can be made closed when the phase angles are equal.

Advantages of Parallel Operating Alternators

• When there is maintenance or an inspection, one machine can be taken out from service and the other alternators can keep up for the continuity of supply.
• Load supply can be increased.
• During light loads, more than one alternator can be shut down while the other will operate in nearly full load.
• High efficiency.
• The operating cost is reduced.
• Ensures the protection of supply and enables cost-effective generation.
• The generation cost is reduced.
• Breaking down of a generator does not cause any interruption in the supply.
• Reliability of the whole power system increases.