Parallel Operation of Alternators - Conditions and Advantages

The method of connecting an alternator or synchronous generator in parallel with another alternator or synchronous generator or with common bus-bars is known as parallel operation of alternators or synchronization of alternators.

Necessity of Parallel Operation :

In most of the power generating stations, instead of having a single large unit (alternator), several small units are used. Also, there are several power stations in the country. By interconnecting all the alternators in parallel connection the alternators form a single large unit and supply to different loads.

A parallel operation with a large number of alternators interconnected is referred to as an infinite bus-bar. An infinite bus-bar exhibits the characteristics like the constant terminal voltage, very small synchronous impedance, and constant bus-bar frequency. Even though it is cheaper to install one or two large units to supply the load, there are a number of good reasons for this practice.

Advantages of Parallel Operation :

The following are the advantages of operating several units in parallel to supply a common load over a single large unit.

• When the number of small alternators is operated supplying a common load. By scheduling the maintenance and repair work of individual generators one after the other, the continuity of the supply can be maintained. In the case of a single large alternator, the supply is interrupted if the continuity of supply is the first priority.
• It is uneconomic to run a large alternator on low loads. By operating several small units we can either disconnect or reconnect alternators from the parallel operation depending upon the load requirement.
• It is easy to install a new additional alternator with the existing parallelly operated alternators when the load demand increased behind the existing capacity. Thereby decreasing the initial capital cost compared to the cost required to buy a new single large alternator in anticipation of increased load demand.
• The machines will operate at their rated capacity, thereby efficiency increases.
• If there is an internal fault in the alternator. The respective alternator gets isolated while the remaining alternators will be operated without interrupting the power supply.
• Due to the limitation of the capacity of alternator that can be built. A power station with a single large alternator cannot meet the requirement of large load demand (more than 1200 MVA). Such a large alternator cannot be built due to physical and economic considerations.

Conditions for Parallel Operation of Alternators :

It is not advisable to connect a standstill alternator to another alternator or live bus-bar directly. This is because the stator induced emf being zero will result in short-circuit. The process of properly interconnecting the alternator (incoming alternator) with other existing alternators (running alternators) operating parallelly or with infinite bus-bars is known as 'Synchronization of Alternator'.

For proper synchronization of alternators, the following conditions must be satisfied,

• The magnitude of terminal voltage generated by the incoming alternator must be the same as that of running alternators or bus-bars.
• The frequency at which the voltage is generated of the incoming should be the same as that of bus-bars.
• The phase sequence of the incoming alternator voltage must be identical to the phase sequence of bus-bars voltage i.e., opposite in phase relative to the local circuit through the armatures and bus-bars.

The failure of any condition from the above causes to flow circulating currents (unusual flow of current) in the incoming alternator and causes losses by overheating alternator parts. The above conditions for synchronization can be achieved by,

1. The voltage is made equal by varying the field excitation.
2. By controlling the prime mover speed driving the alternator frequency can be controlled.
3. To equalize the phase sequence of the incoming alternator, mostly two methods are used.
   • Synchronization by lamps method.
   • Synchroscope synchronization of the alternator.