How To Synchronize Two Diesel Generators for Safe Paralleling

When a project needs more power, adding a second generator may seem like a simple solution.

But two diesel generators cannot simply be connected together and switched on at the same time.

Before generators can operate in parallel, they must first be synchronized.

This process ensures that the generators are electrically compatible before they are connected to a common busbar. Without proper synchronization, paralleling generators can cause severe electrical and mechanical stress, unstable operation, or even equipment damage.

So, what exactly needs to happen before two diesel generators can work together?

What Is Generator Synchronization?

Generator synchronization is the process of matching the electrical characteristics of one diesel generator with the live busbar or another operating generator before closing the circuit breaker.

The main parameters that need to be matched are:

• Voltage

• Frequency

• Phase sequence

• Phase angle

In simple terms, the incoming generator must be “in step” with the existing power source before it is connected.

Think of two runners trying to run side by side. If one is moving faster or in a different direction, they cannot smoothly move together. Generators work in a similar way.

Before paralleling, their electrical output must be properly aligned.

1. Voltage Must Match

The voltage of the incoming generator should be close to the voltage of the busbar.

If there is a significant voltage difference when the breaker closes, a large circulating current can occur between the generators.

This can place unnecessary stress on:

• Alternators

• Circuit breakers

• Cables

• Busbars

• Protection systems

The automatic voltage regulator (AVR) plays an important role in controlling generator voltage and helping the system reach the required synchronization conditions.

2. Frequency Must Match

Frequency is another critical parameter.

For a typical 50 Hz system, the incoming generator must operate at approximately the same frequency as the busbar. The same principle applies to 60 Hz systems.

Engine speed directly affects generator frequency. Therefore, the generator's governor must adjust engine speed so that the frequency approaches the busbar frequency.

If the frequencies are significantly different, the phase angle will continuously change, making it impossible to achieve a stable synchronization point.

3. Phase Sequence Must Be Correct

Phase sequence is often overlooked but is extremely important for three-phase generators.

For example, a system may use an A-B-C phase sequence. The incoming generator must have the same phase rotation.

If the phase sequence is incorrect, connecting the generator to the busbar can result in serious electrical faults.

This is why phase sequence should be checked during commissioning and before paralleling generators.

4. Phase Angle Must Be Aligned

Even when voltage and frequency are correct, the generators still need to reach the appropriate phase relationship before the breaker closes.

The synchronization controller continuously monitors the phase angle between the incoming generator and the busbar.

When the synchronization conditions are within the configured limits, the controller can send a signal to close the generator circuit breaker.

This is the critical moment when the generator officially joins the parallel system.

Automatic Synchronization

Modern generator paralleling systems normally use an automatic synchronization controller rather than relying on manual operation.

The controller continuously monitors parameters such as:

Voltage → Frequency → Phase Angle → Synchronization → Breaker Closing

If the conditions are not correct, the controller will prevent the breaker from closing.

Once synchronization is achieved, the generator can be connected to the common busbar and begin sharing the load.

For multiple-generator systems, the controller can also coordinate load sharing, allowing the available electrical load to be distributed between generators according to the system configuration.

What Happens After Synchronization?

Synchronization is not the final step.

After the generators are connected in parallel, the system must control how they share the load.

Depending on the application, the paralleling system may manage:

• Active power sharing

• Reactive power sharing

• Generator start/stop sequence

• Load demand

• Frequency control

• Voltage control

• Fault protection

For example, if the electrical demand increases, an additional generator can be started and synchronized with the existing system.

When demand decreases, one generator may be disconnected while the remaining unit continues supplying the load.

This provides greater flexibility and redundancy than relying on a single large generator.

Why Paralleling Generators?

Generator paralleling is particularly useful when power demand is large or variable.

Common applications include:

• Data centers

• Hospitals

• Mining projects

• Construction sites

• Industrial facilities

• Large commercial buildings

• Rental power systems

Instead of installing one extremely large generator, several smaller units can work together.

This can provide scalability, redundancy, easier maintenance, and operational flexibility.

For rental companies and distributors, paralleling systems can also provide more flexible power solutions for projects with changing load requirements.

What If Synchronization Is Incorrect?

Improper synchronization should never be treated as a minor issue.

Closing a generator breaker when voltage, frequency, phase sequence, or phase angle conditions are incorrect can create high electrical currents and severe torque on the generator shaft.

Potential consequences include:

• Alternator damage

• Circuit breaker trips

• Protection system activation

• Mechanical stress

• Electrical instability

• Unexpected power interruption

That is why professional commissioning, correct controller configuration, and proper testing are essential before a paralleling system is put into service.

The Key Takeaway

Two generators working together is not simply a matter of connecting two power cables.

They must first speak the same electrical language.

• Voltage must match.

• Frequency must match.

• Phase sequence must be correct.

• Phase angle must be synchronized.

Only after these conditions are satisfied should the generator breaker close and the unit enter the parallel system.

For a reliable paralleling solution, generator hardware is only one part of the system. The engine, alternator, AVR, governor, controller, protection system, and synchronization strategy must all work together.

At POWERZOO, generator set paralleling solutions can be configured according to different project requirements, helping customers build scalable and reliable power systems for demanding applications.

**The goal is not simply to make two generators run.

The goal is to make them work together.**