Parallel Operation of Alternators

Actually, an Alternator is an AC Generator. Due to Faraday’s law of Induction, an EMF is Generated in an Alternator’s stator (Stationary wire) when a Rotating Magnetic field (rotor) is present. It is also referred to as a Synchronous Generator because of the Synchronous speed at which the field poles rotate.
We can talk about Parallel Operation of Alternators here. The Alternators should also be connected in parallel when the AC power systems are Interconnected for Efficiency. In generating stations, there will be more Alternators connected in parallel than two.

Condition for Parallel Operation of Alternator

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

• Synchronizing is the process of Parallelizing two Alternators or an Alternator and an Infinite bus bar system.
• Running machine is the machine which carries the load.
• The Alternator or other machine that must be connected in parallel with the system is an Incoming machine.

The conditions to be satisfied are.

1. The bus bar voltage and the Incoming machine voltage should have the same phase sequence.
2. The incoming machine should have the same RMS line voltage (terminal voltage) as the bus bar or currently operating machine.
3. The phase angle of the two systems should be equal.
4. The frequency of the bus bar and the incoming machine voltages should be nearly identical. When frequencies are not quite equal, large power transients will happen.

Current and power surges will form when the aforementioned conditions are not met. Additionally, it causes an unintentional electro-mechanical oscillation of the rotor, which damages the apparatus.

General Procedure for Paralleling Alternators

A running power system is paralleled with an alternator (generator 2) in the illustration below (generator 1). These two devices are getting ready to synchronise so that they can power a load. A switch, S1, will help parallelize Generator 2 soon. Without meeting the aforementioned requirements, this switch shouldn’t ever be turned off.

1. to equilibrate the terminal voltages. This can be achieved by using voltmeters to adjust the terminal voltage of the incoming machine to make it equal to the line voltage of the operating system.
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.

1. The incoming and running system frequencies must then be checked and verified. It ought to be nearly identical. Examining the frequency of lamp dimming and brightness changes will help with this.
2. The phase will gradually change as the two voltages—those from the running system and the incoming alternator—get closer to being equal in frequency. When the phase angles are equal, these changes can be seen, and switch S1 can be closed.

Advantages of Parallel Operation of 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.