Synchronization of Alternators- Conditions & Advantages

Synchronization of Alternators:

The process of connecting an alternator into grid in parallel with other alternators without any interruption is known as synchronization of alternators. Before going to the explanation of synchronization of Alternators (AC generator), it is important to know that why it is necessary to connect alternators in Parallel.

Why Alternators are Connected in Parallel?

In general, various power systems are connected to grid system. Let assume that the total capacity of interconnected system is 30,000MW while the capacity of largest alternator is 500MW. Since the output delivering capacity of single alternator is very small as compared to the total interconnected power capacity. Hence we cannot supply all energy demand with a single alternator. For that reason, we use multiple alternators to generate the require voltage by connecting them in parallel.

When the load on system is high or load needs more current, you want to keep the voltage same and increase the supply current. Parallel operation of alternators will helps you to do that. When the demand of current increases, you can turn on all the alternators. Similarly, you can disconnect some of the alternators when the load on system decreases. These are some of the specific reasons to operate the alternators in parallel. In order to understand the synchronization of alternators, let discuss about the infinite bus bar.

Infinite Bus Bar:

Infinite bus bar is a large power system (large number of parallel connected alternators) whose voltage and frequency remains constant regardless of the variation in load is known as infinite bus bar. Alternators operating parallel in a power system treated as an infinite bus bar .

Conditions for Synchronization of Alternator with infinite Busbar:

In order to synchronize the alternator with infinite busbar, following conditions must be met for proper parallel operation of alternators.


Phase Angle


Phase Sequence


The rms terminal voltage of incoming alternator must be same as that of the running power system.

Phase angle:

Phase angle between the voltage produced by incoming alternator and bus bar must be zero. In simple words, phase difference between both the voltages is zero (they are in phase with each other.)


Frequency of incoming alternator must be same as that of the bus bar frequency.

Phase Sequence:

Phase sequence of incoming alternator and bus bar must be same. In simple words, identify each phase (1,2,3) of the incoming alternator and ensures that each phase connects with the same phase as that of the busbar. When alternators are connected in parallel in such a way that the above specific conditions are satisfied, it is called synchronizing.

Synchronization of alternators with infinite bus bar

The above diagram shows the synchronization of alternators with infinite bus bar. Alternator which is to be synchronized with infinite bus bar is called incoming machine (G2,G3 in the above diagram). Moreover, a stationary alternator is not synchronized with live bus bar. Since induced emf is zero at standstill condition due to which short circuit will occur in the stator terminals.

Advantages of Synchronization of Alternator:


Load demand is not constant all the time. For that reason, we can turn off some of the alternators during light loads so that other alternators operate at nearly full load with high efficiency.(since alternators operates more efficiently when operates at full load). Similarly, we can turn on all alternators when power demand is large. Hence alternators can be added or turn off according to the demand of load which allow them to operate efficiently.

Continuity of Supply:

When one alternator requires repairing or there is schedule maintenance and inspection, that machine can be taken out from the service while other alternators maintain the continuity of supply. In this way, maintenance and repairing doesn’t require the shut down of system and continuity of supply is maintained.


Multiple alternators can supply a large load than single alternator. Moreover, using 10 alternators (each 20mva) is more economical than using a single alternator of the same rating (200mva). Alternators connected in parallel is also cheaper than running a single alternator.

Load Sharing of Alternators:

Parallel operation of alternators maintains the reliability and stability of power supply. If there is a breakdown of one alternator, there will be no interruption in the power supply. Several units are connected in parallel to the bus bar (power grid) in order to withstand large variations in loads.

Constant Voltage and Frequency:

In a large power system, several generators are connected in parallel forms a system which is treated as an infinite bus. Any change in the operating condition of one alternator doesn’t cause any effect in the frequency and voltage of system. In other words, infinite bus maintains a constant voltage and frequency regardless of the variation in load or individual generator emf.

Behavior of Alternator on Infinite Bus Bar:

One thing to be kept in mind that the behavior of alternator connected to infinite bus bar is quite different as compared to that when it is connected to another alternator in a way that both are parallel. Characteristics of alternator connected to infinite bus bar is as follows:

Effect of Excitation:

When two alternators are connected in parallel, change in the excitation affects the terminal voltage of alternators. However, this behavior of alternator differs when it is connected to infinite busbar.

Change in excitation of alternator connected to infinite bus bar affects the operating power factor of alternator and not the terminal voltage. Increase in the field excitation of alternator causes the supplied reactive power by it to increase and the corresponding decrease in reactive power supplied by other alternators. Hence change in field excitation controls the reactive power (KVAR) of alternator in case of infinite busbar.

Effect of Mechanical Input:

Active power supplied by an alternator depends on the mechanical input power. An increase in the mechanical power to the prime mover of alternator increases the active power (KW) of alternator. This will simply results in the corresponding decrease of active power delivered by other alternators and vice versa. Hence change in the driving torque of prime mover changes the active output power of alternator in infinite busbar.

From the above discussion, we can say that change in mechanical input affects the active power of alternator. On the other hand, change in excitation controls the operating power factor (reactive power) of alternator.(in case of alternator connected to infinite busbar)


Large number of alternators connected in parallel treated as an infinite bus whose voltage and frequency remains constant regardless of the variations in load. The purpose of synchronization of alternators is to share the load between multiple generators. Moreover, active and reactive power (KVAR) of  alternator operating in an infinite bus is controlled by controlling the mechanical input power and field excitation respectively.


That’s all. Hope this will helps you.

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