Parallel Operation of Alternators

Parallel operation of alternators is the process of connecting two or more alternators or synchronous generators in parallel to supply electric power to a common load. The process of matching parameters of parallel connected alternators, such as supply voltage, frequency, phase sequence, phase shift, etc., is called paralleling of alternators.

In this article, we will explore various concepts related to paralleling of alternators and synchronization.


What is the Parallel Operation of Alternators?

When two or more electric generators or alternators are connected together to supply electric power to a load, it is called paralleling of the alternators. The paralleling of electric alternators is required because the electrical loads are unpredictable, and they can change at any time. Therefore, it is necessary to connect multiple alternators in parallel to supply large electrical loads.

However, the paralleling of alternators requires their proper synchronization. Synchronization of alternators is a process of matching all the important parameters of parallel connected alternators.

Conditions for Parallel Operation of Alternator

The following conditions must be satisfied for the parallel operation of alternators.

(1). Same Frequency:

The frequency of alternators to be connected in parallel and synchronized must be the same. This can be achieved by adjusting the speed of the prime movers of the alternator.

If the frequency of alternators operating in parallel is different, it will cause rapid acceleration and deceleration of prime movers and cause instability of the power system operation.

(2). Same Voltage:

The RMS value of voltages generated by all the alternators connected in parallel must be the same. Otherwise, the difference in the voltages of alternators can disturb the flow of reactive power in the electric grid. For example, if the voltage of the incoming alternator (newly added alternator to the existing grid alternator) is greater than that of the electric grid voltage, then the newly added alternator will inject a high amount of reactive power into the grid. On the other hand, if the voltage of the incoming alternator is lower than the grid voltage, then the alternator will absorb very high reactive power from the grid.

Therefore, the voltage of all the alternators operating in parallel must be the same for balancing the reactive power flow in the grid.

(3). Same Phase Sequence:

The phase sequence of all the alternators connected in parallel must be the same. The mismatch of the phase sequence can produce mechanical and electrical stress in the power system.

(4). Same Phase Angle:

Another important requirement of synchronization of alternators is the same phase angle, i.e., all the alternators operating in parallel must not have phase differences. Therefore, before operating two or more alternators in parallel, it must be ensured that the voltage waveforms of all the alternators rise and fall at the same time.

Procedure for Paralleling Alternators

As we know, the process of synchronization or paralleling is nothing but matching alternator parameters such as frequency, voltage, and phase angle.

In practice, a device called a synchroscope is used to perform synchronization of alternators. Also, there is an older method named the “three-bulb method” that can be used to perform synchronization of two alternators operating in parallel.

It is quite important that before operating two alternators in parallel, their synchronization of voltage, phase angle, and frequency must be done.

The diagram below depicts alternator 2 being connected in parallel with a power system that is already running alternator 1. These two machines are preparing to synchronize in order to provide power to a load. In order to parallel alternator 2, a switch called S1 is used. The switch S1 is the circuit breaker. However, it is extremely important that this switch is not closed until all necessary conditions are met.

The procedure of paralleling alternators is as follows.

  1. Check the voltage of running and incoming alternators. The voltage can be adjusted through an automatic voltage regulator(AVR) to increase or decrease the voltage of the alternators. Adjust the voltage and make the voltage of incoming and running alternators equal.
  2. The phase sequence of the alternators’ supply voltage must be the same. The phase sequence of the alternators can be checked by synchroscope and three-lamp methods.
    • The synchroscope does not check the phase sequence. It calculates the phase differences between the supply voltage of incoming and running alternators. The synchroscope pointer rotates if there is a phase difference between power sources. The pointer stops moving and stands vertically right when the voltage sources have the same phase. This indicates that the parallel condition has been satisfied, and breaker S1 can be turned on.synchroscope-method
    • In the three-lamp method, three light bulbs are connected to the terminals of the switch, S1. The voltage across each bulb is the difference in voltage between the incoming and running alternator. Bulbs become bright if the phase difference is large. If the phase difference is large, bulbs will become bright. Conversely, if the phase difference is small, bulbs will become dim. When the phase sequence is the same, the bulbs will show both dim and bright. On the other hand, if the phase sequence is opposite, the bulbs will get progressively brighter. To make the phase sequence equal, you can swap the connections on any two phases on one of the generators.three-lamp-method
  3. Check and verify if the incoming and running system frequencies are nearly the same. One way to do this is by examining the frequency of dimming and brightening of lamps.
  4. When the frequencies of the incoming alternator and running system are almost the same, their voltages will gradually alter in phase. These changes can be observed, and S1 can be closed when the phase angles are equal.

Advantages of Paralleling of Alternators

The major advantages of parallel operating alternators are as follows:

(1) Increased Power Generation – When multiple alternators are operated in parallel, they can generate more electric power that can supply larger electrical loads. Therefore, paralleling multiple alternators is an economical way of generating large electric power in the case of high-demand conditions.

(2). Improved Load Sharing Capability and Flexibility – When multiple alternators are connected in parallel, the total load will be distributed across all the alternators, which reduces the overloading of a specific alternator. This optimizes the fuel consumption and improves the overall power generation efficiency.

(3). Improved Reliability – When multiple alternators are connected to operate in parallel, and one of them fails or is isolated for maintenance, then the remaining alternators can operate without disturbing the power supply continuity. Hence, paralleling or synchronization of alternators increases the redundancy and reliability of the power generation system.

(4). Easy Maintenance – Paralleling or synchronization of alternators makes their maintenance easier. We can take out any alternator without affecting the power supply continuity.

(5). Higher Scalability – Paralleling of alternators allows for a high degree of scalability in power alternators. We can increase the power generation simply by synchronizing more alternators with the existing grid.


In conclusion, this is all about the paralleling of alternators and synchronization. Parallel operation of alternators is performed to increase the power generation capacity, improve efficiency, load sharing optimization, increase reliability, etc.

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