The synchronization of generators in parallel arises as a need to supply large demands of energy that would be impossible to supply with a single generator. Parallel synchronization also applies to those installations that require a much greater reliability and safety mechanism, since having more than one generator operating at the same time is more complicated to fail or fail in unison, thus ensuring continuity, At least partially, of the electricity supply.

The parallel connection of two or more generators involves a series of complex operations that allow the machine to be synchronized. Let’s see below what checks should be made before synchronizing the generators and the different methods that exist to do so.

Conditions required for parallel synchronization

Connecting the generators in parallel arbitrarily would possibly cause severe damage and the load may lose power, so that to make the parallel connection of two or more generators the following aspects must be taken into account.

Equal Voltages

If the generator voltages are not exactly the same, there will be a very large current flow when the switch is closed. To avoid this problem, each of the three phases must have exactly the same magnitude of voltage and phase angle as the conductor to which it is connected.

And also make sure that the RMS line voltages are equal.

Equal frequencies

The frequencies of the 2 or more generators as well as the voltages must be the same as they would cause serious problems.

Phase Sequences

All generators must have the same phase sequence.

The phase angles of all phases must be the same, the sequence in which the phase voltage reaches its peak in all generators must be the same.

Similar constructive characteristics

A recommended factor that greatly facilitates synchronization, is that the machines must be the same, it is obvious that it will not be possible to obtain two totally identical generators, but unless they look a lot in their constructive part, besides that, the powers which each deliver is of similar value, and preferably of the same manufacturer.


Procedure for connecting the generators

In this point commonly called paralleling of generators, some steps and precautions must be followed for the optimal operation of these synchronous machines. 

Start the generator to be connected

At this point the generator is put into operation to be placed in parallel this involves carrying it at its synchronous speed and equalizing the value of its voltages. Another condition is the phase sequence of the generator in approach should be compared to the phase sequence of the system in operation.

To ensure compliance with these conditions and those set forth in previous points, some methods are indicated below:

Phase sequence

There are many ways to check this one is to alternatively connect a small induction motor to the terminals of each of the two generators.

  • If the motor rotates in the same direction both times, then the phase sequence is the same on both generators.
  • If the motor rotates in opposite directions, then the phase sequences are different and two of the conductors of the generator must be inverted.

Another simpler method to measure the sequence is the use of a sequencer, which can be either electronic or an electromechanical, but both follow the same principle of the engine discussed above.

Synchronization of generators

This point is the most important when it comes to generators in parallel, that is why before putting the two or more generators in operation we must do some tests to ensure their correct synchrony.

The method of the “phase lamps off”

Tecnics Grupos Electrogenos - Sincronización método lámaparas apagadas.
Synchronization: method of the Lamps off

This method consists in connecting the lamps between AA’, BB’, CC’ the potential difference between lamps, it indicates if the conditions are fulfilled ie when the lamps are off the conditions are verified.

The method of “rotating or lit lights”

Tecnics Grupos Electrogenos - Sincronización método lámparas apagadas.Synchronization: method of the Lamps on

The potential differences between lamps vary in modulus if the rotational speeds are different. When they are in synchronism the lamp AA ‘is off and the other two shine equally, otherwise this is seen alternately in one direction or another as if turning, indicating that the machine goes slower or faster. Once the conditions are met it can be said that the machines are synchronous.

In previous cases it is also advisable to use a voltmeter called voltmeter zero, which is placed between the same phase and gets its name because when the voltmeter marks a value of zero volts the generator is in sync.

Using a Synchronoscope

Without a doubt, the simplest method is to have a synchronoscope, which, as is supposed to be an instrument that indicates the synchrony of the generators, measures the difference in the phase angles of any phase between the two systems. Analog needle and electronic models can be found.

Automatic synchronization, programmed digital controllers

Nowadays, and with the technological advances that have been incorporated in the sector, the previous synchronization methods have become obsolete.

Most generator sets, during the manufacturing process of their equipment, install digital control modules. These modules require pre-programming performed by a specialized technician, and once programmed, they perform and supervise the synchronization of the different groups, analyzing and controlling in real time all the parameters of the generator set, so that the whole synchronization operation is performed automatically and safely.

Optionally, and for critical installations additional external controllers can be added to improve security.

These external equipment are responsible for analyzing critical processes of synchronization, inputs and output to load, load balancing, etc., of the installation, and work together with the digital controllers of each equipment, so that the programmed and automated starts and stops, according to the needs of the installation to be supplied, are performed automatically, with no worries for the end user.


The synchronization of generators in parallel brings great advantages as we discussed at the beginning of the article.

  • It allows to supply large amounts of energy that would otherwise be very expensive or unviable in many cases.
  • Provides greater security to the installation, since having several equipment, it is easier, repair, replace, etc., one of them, without having to completely drop the system, since the rest of the equipment continue to operate .
  • At times it can be much more economical to install several less powerful equipment with a synchronous installation in parallel than a single generator much more powerful.
  • The synchronization of generators in parallel allows a greater efficiency of the equipment, since in this type of installation it is possible to connect and disconnect one or more of the synchronized generators according to the demand of the electric network, which allows to take advantage of the Maximum power of the group thus reducing consumption.


In Tecnics Grupos Electrógenos, S.L. We have more than 50 years of experience in the sector, we have extensive experience in installations synchronized with generators in parallel.

Consult us before placing your installation!