Electric Generator Synchronization: All You Should Know

All critical loads in my work are working on a generator, and another one is standby in parallel connection with it.

We are doing this to make sure that the load will not stop when we do periodic maintenance.

All we need is to start the stand by generator, check the synchronization panel to make sure all parallel connection conditions are valid, then we start the stand by generator and after few seconds we stop the operating one.

Now, the generator is ready for making what ever we need to make. In this article I will discuss generator synchronization. Let’s get started.

What is generator synchronization?

synchroscope meter for generator synchronization
synchroscope meter for generator synchronization

Synchronization of Generators is known as synchronizing variables like frequency, voltage, phase angle, and phase sequence of an alternator or any other sources with a healthy operating power system or with other generator to be able to connect them in parallel.

Generators cannot supply electricity to the electrical power systems unless their frequency, voltage, and other variables align with the network. Synchronization is accomplished by controlling the exciter current and the generator’s engine speed.

In particular, the need for synchronization comes when more than two alternators work together to provide energy for the loads. Connecting several alternators operating in parallel to provide larger loads is essential.

In most cases, when generators are synchronized, the synchronization procedure is controlled by an automatic synchronizer with manual control capabilities that can be utilized for backup scenarios.

Synchronizing panels typically show changes the operator must make concerning when it’s appropriate to shut off the circuit breaker.

What is condition for parallel operating of Generator?

Specific requirements must be fulfilled for the successful parallelization of alternators.

The following conditions must be met in order to sync a generator with the grid or with other generators.

Phase Sequence

Three phases in the alternator linked to the power system bus must be similar to all three phases of bus bars (or electrical grid).

This problem arrives especially in the circumstance of initial installation or after maintenance.

Voltage Magnitude

Incoming alternator RMS voltage must be the same as a bus bar or electric grid RMS voltage.

If the voltage of the alternator that is incoming exceeds the bus bar’s voltage, there will be a significant reactive power flow through the generator and into the grid.

If the alternator’s voltage is less than the bus bar’s voltage, the generator immerses high reactive power from the bus bar.


The incoming generator’s frequency must match the bus bar’s frequency.

A mismatch in frequency could result in high acceleration as well as slowing of the primary mover, which could cause an increase in the torque of its transients.

Phase Angle

The phase angle between the generator’s voltage and the voltage from the bus bar must be zero.

This can be verified by comparing the appearance of zero crossings or high points of signal wave forms of the voltage.

Advantages of parallel operation of generators

Redundancy and reliability are increased

Enhancing the reliability and redundancy of critical and non-critical loads. The system makes sure that it can supply constant power to critical loads.

If one of the generators malfunctions, power will be provided by the second or a parallel unit.

Low cost of power generation

Generators’ cost rises according to the size of the generator and is higher for models with ratings greater than 600KW.

This is because there is a market that caters to smaller engine sizes more and, therefore, more manufacturers, resulting in lower prices. Therefore, utilizing smaller generators can be cheaper than one big generator.

Reduced light load on the prime mover

In most instances, loads fluctuate between times, and it is normal to have a generator operating at 30% capability when the demand of load is lower.

This could result in wet stacking. The efficiency of the primary mover is usually greater with loads that are between 75% to 100%.

A light load implies that the generator operates inefficiently. A smaller generator can increase efficiency, which results in cost reduction.

More control and savings on generating costs

The power generated by various small generators is equivalent to the energy produced by one big unit.

But there is more control and balance in smaller generators. According to the load, the load can be balanced across different circuits and choose the capacity to produce at any time.

Parallel systems can result in enormous savings if all generators operate over 75percent of the nominal load. This is the level at which generators run on the least fuel.


The utilization of multiple generators allows you to provide a variety of loads without having to pile up expensive units or spending too much money on an enormous generator that’s capacity is seldom utilized.

Moreover, generators can be added gradually when demand rises.

What is synchronization panel?

synchronization panel
synchronization panel

Synchronization panels are typically developed and utilized to meet the power system’s requirements.

They work both manual and also with an automatic function of synchronizing two or more breakers or generators.

They are commonly used in the process of synchronizing generators, as well as they are also used to provide multiplex solutions.

These load-sharing units continually check the load. During low demand, one or two generators may be closed to conserve fuel. The third and second generators will be re-started, then synchronized, and connected to load when demand increases.

How to synchronize two generators?

Although all modern control panels, whether on ships or in large industries, can parallelize generators automatically, it is essential to know and prepare to accomplish it manually.

 Manual Synchronization:

There are many techniques available to synchronize alternators. The most commonly used methods for synchronizing the alternators are listed below:

  • Three dark Lamps Method
  • TwoBright and One Dark Lamp Method
  • Synchroscope Method

Three dark Lamps Method:

Assume that the alternator connects to the load, which supplies the rated frequency and voltage. Alternator-2 will be joined in parallel to alternator-1.

  • Three light lamps (each being tested for the voltage of the alternator’s terminal) are connected to the switches in the alternator-2. The moment that all requirements of parallel operation are met, the lamps will be in more or less dark.
  • To ensure that the alternator-2 stays synchronized adequately to that of the bus bar, the primary mover of the alternator-2 is operated at a rate similar to the synchronous speed as determined by the bus bar’s frequency and the number of poles that the alternator has.
  •  Generator 2 field current is increased until the voltage across the machine’s terminals is at the same level as the voltage of the bus bar (by taking the readings from voltage meters).
  • If the lamps turn ON and off simultaneously, the alternator-2’s phase sequence is in line with the bus bar. However, if they switch on and off in succession, it resembles the incorrect phase sequence.
  • If you alter the connections of both leads on the alternator-2 after the machine’s shutting down, the sequence of the phases can be changed.
  • Based on the frequency difference between alternator-2 and bar voltage, these lamps’ ON and off rates are determined. Thus, flickering frequency needs to be reduced to correspond with the frequency. This is done by adjusting the speed of the alternator with the control of its primary mover.
  • Once all the parameters are set, the light bulbs turn dark, and the synchronizing switch may be closed to allow alternator-2 and alternator-1.
  • The biggest drawback to using this technique is that the rate of flickering only shows the variation between the alternator-2 and the bus bar However, information regarding alternator frequency in relation to bus bar frequencies isn’t readily available through this method.
  • If you have a bus bar with a frequency of 50Hz, then the frequency of the lamps’ flickering is the same regardless of whether it is 49 or 51 Hz because the difference between these two instances is 1Hz.

Two Bright and One Dark Lamp Method

  • In this case, the lamp L2 is connected to the pole located on the middle line on the synchronizing switch like the dark lamp technique; in this case, lamps L1 and L3 are linked in a reversed manner.
  • The method of checking voltage is the same as the previous method, but after that, the lamps are dark and bright in succession. The higher or lower value of the frequency of the alternator in relation to bus bar frequency can be determined using the order that the lamps change from bright and dark.
  • The sequence of getting bright and dark from L1 – L2 – L3 shows that the generator’s frequency is greater than the bus bar’s frequency. Therefore, the speed of the alternator must be reduced using prime mover control until speed of flickering decreases to a minimum.
  • On the other side, the sequence that flickers L1-L3-L2 shows that the frequency of the alternator is lower than the frequency of the bus bar.
  • This means that it is possible to increase the speed of the alternator with the help of the primary mover until the speed of flickering is as low as it is. The synchronizing switch is locked if lamps L1 and L3 are identically bright. The lamp L2 appears dark.
  • The downside of this approach is that the accuracy of the phase sequence is not verified. However, this is not required for permanently connected alternators, in which checking the phase sequence is sufficient to carry out the first time of operation by itself.

Synchroscope Method

synchroscope meter for generator synchronization
synchroscope meter for generator synchronization
  • It’s like the two bright and one dark lamp technique and shows whether the frequency of the alternator is more significant and lower than that of the bus bar’s frequency. Synchroscopes are used to ensure more precise synchronization. It is composed of two terminals.
  • One terminal’s pair marked as ‘existing’ has to connect to either the bar-bar terminal or the alternator in place. Another terminal’s pair marked “incoming” must be connected to the alternator’s terminals connected to the incoming one.
  • The synchroscope has circular dials, which pivot a pointer that can turn in the clockwise or anticlockwise direction.
  • When the voltage is confirmed, the operator needs to examine the synchroscope. The rotation speed is the frequency that is different between the alternator and bus bar.
  • The direction in which the pointer turns (to either slow or fast) provides the information, which frequency the alternator is supplying is greater or less than the bus bar’s frequency. The pointer can move either quickly or slowly.
  • The correct adjustment must be made to control the frequency of the alternator so that it can make the speed of rotation of the pointer to a minimum. So, synchroscopes and voltage meters are sufficient for the synchronization process. However, in most cases, the combination of lights with a synchroscope can be utilized as a double-check system.
  • These are the techniques for synchronizing generators. The process should be carried out with care to avoid disruptions within the power system and avoid severe device injury. The three-lamp method is not recommended due to its lack of precision in operation and the manual.

Automatically Synchronization

An automatic synchronization unit is being developed to connect two generators in parallel. Generators can be connected easily using the designed control unit.

The frequencies, voltages, phase sequences, and synchronism data have been sent onto the programed microcontroller installed in the automatic synchronizer panel.

These data are then monitored and analyzed using the controlling algorithm programmed to the microcontroller.

Generators that operate in parallel are automatically realized when all conditions for parallel connection are observed matched.

The system does not require additional measuring tools to monitor and control processes. The automated synchronization unit is quick, economical, and reliable. It can be used to monitor measurements and the simultaneous operation of the generators that synchronize.

Load sharing of two parallel generators

Load sharing refers to the proportional distribution of the total load in kVAR and kW between several generator sets in an in-parallel system.

The concept of sharing the load is essential to avoid over-loading and instability issues in the generator sets in the system.

Active Power (KW) load sharing

When generator sets work simultaneously. Each set’s speed regulator determines the proportional share of the total active power (kW) requirements for the set.

This can be achieved by increasing or decreasing the fuel supply to the engine systems. If the fuel for the engine of one set of generators within a group increase, it won’t cause an improvement in the engine’s speed and, consequently, frequency (as the case if it were operating on its own); however, it will cause an improvement in the percentage of total power it delivers.

When the fuel supply is delivered to an engine in one set of generators within a group decrease, it won’t increase speed and frequency (as the case if it was operating by itself); however, it will reduce the total amount kW load it delivers.

The control system of Generator sets (via the control of engine speeds) oversees and regulates the distribution for the total power load according to the ratings of the engines in the generator sets in the system.

Read also KW to HP on line converter

REACTIVE power (KVAR) load sharing

If generator sets are operating in parallel, each generator set’s alternator field excitation system is responsible for the proportional share of the total reactive power needs (kVAR) for the entire system.

The load sharing of the kVAR is accomplished by increasing or decreasing the excitation of the alternators of the system.

When the field excitation of one generator set within an ensemble is increased, i.e., overexcited, it will not result in the voltage increase (as it would do if it were operating on its own); however, it can cause an increased amount of total power that it will produce and a reduction in the power ratio.

When the field excitation of a generator set within an ensemble is diminished, i.e., under excited, it won’t result in a decrease in the voltage (as it would have if it were operating on its own); however, it will result in an increase in the percentage to the overall kVAR.

It can increase its power efficiency Unwanted reactive current circulating (cross-current) is generated by the alternators when their voltage isn’t in sync with it.

A voltage control mechanism in Generator sets (via the voltage control system for the alternator system) regulates and monitors the share that the total power kVAR loads in proportion to the ratings of the alternators in the generator sets in the system.

Which is better connecting a generator or two in parallel?

Instead of relying solely on one generator in power outages, facilities managers realize the benefits of parallelizing generators.

Paralleling multiple or two generators can improve efficiency, reliability, load management, and maintenance capabilities. All this with little to no interruption to the power supply.

One of the main advantages of parallelizing two or more generators is eliminating the possibility of failure at a single point in the standby power system, which reduces the chance of losing power during the case of a generator failure.

As a result, paralleling multiple generators will better serve hospitals that offer continuous emergency care and business.

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