Premium Membership ♕

Save 50% on all EEP Academy courses with Enterprise Membership Plan and study specialized LV/MV/HV technical articles & guides.

Home / Technical Articles / 2 types of synchronisers required on a power system with two supplies

Synchronisers are required at points on a power system where two supplies (either generator and grid, or two grid supplies) may need to be paralleled.

2 types of synchronisers required on a power system with two supplies
2 types of synchronisers required on a power system with two supplies (on photo: DEIF's synchroniser, applied for synchronisation of a generator to the busbar and closing of its circuit breaker when the voltage difference, the slip frequency and the phase angles are within the preset limits)

They are more than just a measuring device, as they will provide contact closures to permit circuit breaker closing when conditions for paralleling (synchronising) are within limits.


However, they are not regarded as protection relays, and so are discussed in this article for convenience. There are two types of synchronisers:

  1. Check synchronisers and
  2. Auto-synchronisers

1. The function of a check synchroniser

The function of a check synchroniser is to determine if two voltages are in synchronism, or nearly so, and provide outputs under these conditions. The outputs are normally in the form of volt-free contacts, so that they may be used in CB control circuits to permit or block CB closing.

When applied to a power system, the check synchroniser is used to check that it is safe to close a CB to connect two independent networks together, or a generator to a network, as in Figure 1.

In this way, the check synchroniser performs a vital function in blocking CB closure when required.

Synchronism occurs when two a.c. voltages are of the same frequency and magnitude, and have zero phase difference. The check synchroniser, when active, monitors these quantities and enables CB close circuits when the differences are within pre-set limits.

Check synchroniser application to generator
Figure 1a – Check synchroniser application to generator

While CB closure at the instant of perfect synchronism is the ideal, this is very difficult to obtain in practice and some mismatch in one or more of the monitored quantities can be tolerated without leading to excessive current/voltage transients on CB closure.

The check synchroniser has programmable error limits to define the limits of acceptability when making the comparison.

Check synchroniser application to two networks
Figure 1b – Check synchroniser application to two networks

The conditions under which a check synchroniser is required to provide an output are varied. Consider the situation of a check synchroniser being used as a permissive device in the closing control circuit of a CB that couples two networks together at a substation.

It is not sufficient to assume that both networks will be live, situations where either Line A or Busbar B may be dead may have to be considered, leading to the functionality shown in Table 1(a).

Table 1 – Check Synchroniser function set

1Live bus/live line synchronising
2Live bus/dead line synchronising
3Dead bus/live line synchronising
4Network supply voltage #1 deviation from nominal
5Network supply voltage #2 deviation from nominal
6Voltage difference within limits
7Frequency difference within limits
8Phase angle difference within limits
9CB closing advance time
10CB closing pulse time
11Maximum number of synchronising attempts

When the close signal is permitted, it may be given only for a limited period of time, to minimise the chances of a CB close signal remaining after the conditions have moved outside of limits.

Similarly, circuits may also be provided to block closure if the CB close signal from the CB close controls is present prior to satisfactory conditions being present – this ensures that an operator must be monitoring the synchronising displays and only initiating closure when synchronising conditions are correct, and also detects synchronising switch contacts that have become welded together.

A check synchroniser does not initiate any adjustments if synchronising conditions are not correct, and therefore acts only as a permissive control in the overall CB closing circuit to provide a check that conditions are satisfactory.

In a substation, check-synchronisers may be applied individually to all required CBs.

Alternatively, a reduced number may be installed, together with suitable switching arrangements in the signal input/output circuits so that a single device may be selected to cover several CBs.

Go back to contents ↑

2. Auto-synchroniser

An auto-synchroniser contains additional functionality compared to a check synchroniser. When an auto- synchroniser is placed in service, it measures the frequency and magnitude of the voltages on both sides of the circuit breaker, and automatically adjusts one of the voltages if conditions are not correct.

Application of auto-synchronisers is normally restricted to generators – i.e. the situation shown in Figure 1(a), replacing the check synchroniser with an auto- synchroniser.

This is because it is generally not possible to adjust either of the network voltages by changing the settings of one or a very few equipments in a network. When applied to a generator, it is relatively easy to adjust the frequency and magnitude of the generated voltage by transmitting signals to the Governor and AVR respectively.

The auto-synchroniser will check the voltage of the incoming generator against the network voltage for compliance with the following:

  1. Slip frequency within limits (i.e. difference in frequency between the generator and network)
  2. Phase difference between the voltages within limits
  3. Voltage magnitude difference within limits

The CB close command is issued automatically when all three conditions are satisfied. Checks may also be made that the network frequency and voltage is within pre-set limits, and if not the synchronising sequence is locked out. This prevents synchronising under unusual network conditions, when it may not be desirable.

Auto-synchronizer application diagram
Auto-synchronizer application diagram (SELCO’s T4500 Auto Synchronizer provides automatic synchronization of an incoming generator to a busbar in a minimum of time, by controlling the speed via the electric servomotor on a conventional speed governor, or by controlling an electronic speed controller via an intermediate motorized

This facility should be used with caution, since under some emergency conditions, it could block the synchronising of a generator that was urgently required in service to help assist in overcoming the condition.

If (a) above is not within limits, signals are sent automatically to the governor of the generating set to adjust the speed setpoint appropriately.

In the case of (c) not in limits, similar signals are sent to the Automatic Voltage Regulator to raise or lower the setpoint. The signals are commonly in the form of pulses to raise or lower the setpoint, but could be continuous signals if that is what the particular equipment requires.

It is normal for the speed and voltage of the generator to be slightly higher than that of the network, and this can be accommodated either by initial settings on the Governor/AVR or by providing setpoint values in the synchroniser. This ensures stable synchronising and export of power at lagging power factor to the network by the generator after CB closure.

The possibility of tripping due to reverse/low forward power conditions and/or field failure/under-excitation is avoided. Use of an auto-synchroniser also helps avoid human error if manual synchronising were employed – there is potential for damage to equipment, primarily the generator, if synchronising outside of permitted limits occurs.

To ensure that the CB is closed at the correct instant, the CB close time is normally a required data item. The auto-synchroniser calculates from a knowledge of this and the slip frequency the correct time in advance of phase coincidence to issue the CB close command. This ensures that the CB closes as close to the instant of phase coincidence as possible. Upon receipt of the signal indicating ‘CB closed’ a further signal to raise frequency may be sent to the governor to ensure stable export of power is achieved.

Conversely, failure of the CB to close within a set time period will reset the auto-synchroniser, ready for another attempt, and if further attempts are still unsuccessful, the auto-synchroniser will lock out and raise an alarm.

Practice in respect of fitting of auto-synchronisers varies widely between Utilities. Where policy is flexible, it is most common when the time to synchronise is important – i.e. emergency standby and peak lopping sets. Many Utilities still relay on manual synchronising procedures.

It is also possible for both an auto-synchroniser and check-synchroniser to be fitted in series. This provides protection against internal failure of the auto-synchroniser leading to a CB close command being given incorrectly.

Table 2 – Additional functions of Auto-Synchroniser

1Incoming supply frequency deviation from nominal
2Incoming supply voltage raise/lower signal
3Incoming supply voltage raise/lower mode (pulse/continuous)
4Incoming supply frequency raise/lower mode (pulse/continuous)
5Incoming supply voltage setpoint
6Incoming supply frequency setpoint
7Voltage raise/lower pulse time
8Frequency raise/lower pulse time

Go back to contents ↑

Reference // Network Protection & Automation Guide by Alstom

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


    Jul 27, 2020

    Electrical protection and control systems details

  2. Olli Rintamaki
    Aug 28, 2019

    Autosynchronization is not actually limited to generator breakers only. One can find cases where there is a need to synchronize for example bus sectionalizer breaker, where a generator (or number of generators) are connected to one bus section and the grid supply to the other bus section. In this scenario the autosynchronizer must have capability to control the generator, or generators, connected to the first bus section in order match the synchronizing conditions across the bus sectionalizer breaker.

  3. Aurel MICLEA
    Jun 27, 2017


    I want to receive more detail and price list.
    Email adress: [email protected]

Leave a Comment

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

  +  fifty  =  fifty nine

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge