What is parallel operation of generators? 4 major requirements for the operation of parallel generator

What is the paralleling of generators

We use generators as a backup power supply in case of power failures so we can select a generator according to our need of backup power, in certain cases we may need more backup load so the generator output won’t be enough In that case, we can use paralleling generators, it is the process by which two or more generators connected together using special equipment to form a larger capacity power source


What are the requirements for the parallel operation of the Generators

Before connecting any two generators the oncoming terminal unit must have these features

• It must have the same magnitude as the running generator

• It must be in phase with the corresponding phases of the running unit

• Its phase sequence must be the same as the running generator

• It must have an equal frequency as the running generator

If the above requirements are not met then there will be circulating current between the generators which could be large enough to damage the machines

To check the above requirements the following method can be used

One “dark” two “bright” lamps method

In this method a lamp is connected across the a-phase of two generators and the second lamp is connected between the b-phase of the running generator to the c-phase of the oncoming generator, then the third lamp is connected between the c-phase of the running machine and the b-phase of the oncoming machine and by this arrangement the generators are paralleled when one lamp is dark and the other two is bright

Benefits of parallel power generation

In the early days, the parallel system wasn’t used because of its complexity, higher cost, and space. So many industries didn’t use it when they needed it because of these limitations. But now due to the arrival of sophisticated integrated digital control technologies, it is very easy to operate the parallel system and to benefits from its advantages


By using parallel generator we could get more reliability than a single generator unit. we could get the desired load by using the parallel generator so we could get the required load by this and in a certain case, if one unit fails the load can be distributed to the other unit of the system in case of parallel generators but it is not possible in a single unit generator


By using the parallel system one has the ability to expand to his needs. The required load could be received by this without overrunning the budget we can add generators according to the available space and generators can be detached according to our need

Easily serviceable

If one of the generators needs to be serviced it can be dismantled from the unit and serviced thus it won’t interrupt the function of the other loads.


We can use small generators for parallel system and thus we could get the required load and the generator system will fit in the desired space and by using small size generator we could use it in places were space is restricted

What is required to parallel generators

• Synchronizing

• Switching device

• Load sharing

• Protection


• Same phase to phase voltage

• The same number of phases

• Same phase rotation

• Electronic governor- load sharing

• Electronic voltage regulator/ paralleling capability

• Identical internal alternator winding pitch


Switching can be done by using circuit breakers and integrated switching can be done with the utilizing contactor mounted on the generator Load sharing

Load sharing

Isochronous load sharing or speed drop

Reactive power

Reactive cross current or voltage drop


• Synchronizing process

   25 sync check relay

• Real power system (governor & engine)

32 reverse power, frequency protection

• Reactive power system (Regulation & excitation)

Voltage protection

• Cabling and alternator

50/51 overcurrent